Red streaks in a goldfish's fins or body

Red streaks are the goldfish equivalent of a warning light. They show as fine red lines along the rays of the tail and fins, sometimes as a flush at a fin base or a bloody patch on the belly, and in most home aquariums they mean the water is burning the fish. Ammonia and nitrite, the waste products that build up in a tank that is new, overstocked or behind on maintenance, damage the thin tissue of the fins and gills, and the tiny vessels inside show it. Less often the cause is a bacterial infection in the bloodstream, an injury, or simply the visible anatomy of a pale fish.

First: showing through, or coming out?

Settle this before anything else, because the two look similar in a tank and are not the same problem at all.

Blood showing through is what this page is about. The fins and the thin skin of a goldfish are close to translucent, and the vessels running inside them are normally invisible. When the tissue is inflamed or the vessels are leaking into it, those vessels become visible as fine red lines along the rays, or as a flush at a fin base. Nothing is leaving the fish. The blood is inside, where it belongs, and you are seeing it through tissue that has stopped hiding it. Wipe a clean finger along the outside of the glass and nothing changes; look again in an hour and the pattern is the same.

A fin, ray by ray

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Photograph: Bechstein, CC BY-SA 3.0 · Wikimedia Commons. Numbers and lines added.

1. The base. Where the fin leaves the body. Rot that reaches it, or redness spreading from it onto the flank, is no longer a fin problem, and is the time for a vet.
2. The rays. Fine bony rays, each with a blood vessel along it. Red streaks follow them when ammonia or nitrite burns the fish, or when a bacterial infection reaches the blood; faint, even pink lines that never change are normal.
3. The web. The thin skin between the rays. In clean water a torn edge starts to close within days and the web grows out with the rays; regrowing the full length takes months, and longer in cool water.
4. The edge. Where fin rot starts: frayed, ragged or melted, with a white, milky or red rim, and advancing. A split with clean edges is a tear, and heals.
5. Long fins. The longer the fin, the more it catches on decor and the longer it takes to regrow: months, for a long fancy tail.

Blood coming out is an injury, and belongs on a different page. It appears at a torn edge rather than along the rays, it is often a single site rather than a pattern, it may smear or trail briefly in the water, and it stops. Injured goldfish first aid covers wounds, and torn fins covers the damaged edge afterwards.

The one that catches people out is blood at the base of a fin, around the vent, or seeming to seep from under a scale. That reads as a wound and is rarely one: it is the internal picture, usually septicaemia or advanced ammonia damage, and it is more serious than a tear rather than less.

Telling the causes apart

Water poisoning tends to affect the whole tank. The streaks run through several fins, the gills may look dark red or unusually pale, and the fish breathes fast or hangs near the surface. There is nearly always a story behind it: a tank set up in the last few weeks, a filter rinsed under the tap, a new fish, a holiday feeding block, a missed water change. If two or three fish show streaks on the same day, the water is the suspect until proven otherwise.

Septicaemia looks like a sick fish, not just a marked one. The redness spreads beyond the fin rays into blotches on the flanks or belly, and it comes with lethargy, refusing food, and sometimes bloating, bulging eyes or open sores. Usually one fish is far worse than the rest.

An injury is local: one fin, one side, one bruise-like patch on a fish that is otherwise eating and swimming normally. It fades over a week or two, though a damaged fin can become a site for fin rot or fungus if the water is poor.

Normal blood vessels are constant: faint, evenly spaced pink lines along the fin rays that never change from week to week, on a fish that is well. Compare an older photo; if they were always there, they are anatomy.

What to do first

Test for ammonia and nitrite before anything else, and note the pH while you are at it. Everything that follows depends on that one result: a reading above zero makes this a water emergency with a clear sequence, and a reading of zero on a well-maintained tank moves the question somewhere else entirely.

If either reads above zero, work through the next section today. Our water change guide covers the practical side of the changes it asks for, and cycling a goldfish aquarium explains why a new tank goes through this phase at all.

Aquarium salt reduces how much nitrite a fish absorbs through its gills and is a traditional support during a nitrite spike; the right amount depends on the water volume and the fish, so follow a reputable protocol rather than guessing.

The first twenty-four hours

If ammonia or nitrite is reading above zero, this is the sequence. It is worth having in one place, because most people find this page at the point where the fish already looks wrong.

1. Change water, generously. If ammonia reads 2 mg/L or more, change at least half of it; below that, thirty to fifty percent. Use conditioned, temperature-matched water. This is the only thing that removes ammonia and nitrite immediately, and it is the whole treatment. Do it now rather than after reading the rest. The exception is a tank that has gone months without proper changes and has a pH below about 6: a big change lifts the pH in one step and makes the ammonia far more toxic, so change only a little each day there.
2. Stop feeding. Every meal becomes more ammonia within hours, and a healthy goldfish loses nothing by missing several days. This is the second most effective action available and it costs nothing.
3. Add aeration. An air stone, or the filter outlet raised to break the surface harder. A fish with irritated gills extracts oxygen poorly and needs more of it in the water — aeration and oxygen covers the options.
4. Leave the filter alone. Do not clean it, do not replace media, do not rinse anything under the tap. The bacteria in there are the only thing that will end this properly, and a tap rinse is often what started it.
5. Retest in twelve hours, and change water again if either reading is above zero. Daily changes until both are zero, then a few more days for confidence.
6. Find the cause while you wait. A new tank, a cleaned filter, a dead fish behind the decor, a holiday feeder, a jump in stocking, a heater that failed. The readings tell you there is a problem; only the history tells you why.

What not to do in the same period: no medication, no antibacterials, no filter cleaning, no large temperature change, and no complete water change, which removes the fish’s entire chemical environment at once and shocks an already strained animal.

Conditioners that “detoxify” ammonia

Worth explaining properly, because the test kit will appear to contradict the bottle and that is where people panic.

What they do. Products of this kind convert free ammonia into a bound form that gills cannot absorb. The fish is genuinely safer while it holds.

Why your test still reads high. A standard liquid test measures total ammonia and counts the bound form along with the free form. So a tank that has just been dosed can read 2 mg/L while the fish is in far less danger than that number implies. The reading is not wrong; it is measuring something different from what you want to know.

Why it is not a solution. The binding is temporary — it reverts over a day or two — and the filter still has to process the ammonia either way. Treating a bottle as a substitute for water changes is a common way a manageable spike becomes a prolonged one.

So: use it, and keep changing water. It buys hours, which is genuinely valuable at eleven at night with a flushed fish and no clean water prepared. It does not buy days. The source water page also covers the case where the ammonia is coming out of the tap in the first place, which happens more often than people expect.

The gills are the part you cannot see

An important thing on this page after the water change, and the reason a fish that looks better may not be better.

Fins are visible and gills are not, but the gills are where the real injury happens. Ammonia inflames and scars the delicate gill filaments, and nitrite disables the blood’s ability to carry oxygen. Both leave a fish extracting oxygen poorly for weeks after the water has been corrected — long after the red in the fins has faded.

So a fish that is still breathing fast, hanging at the surface, sitting on the bottom or off its food in water that now tests clean is not a fish with a new problem. It is a fish that is recovering, and what it needs is the least eventful few weeks you can arrange:

• Immaculate water, changed regularly, tested weekly.
• Extra aeration, kept up even after the fish looks brighter.
• Light feeding, because digestion demands oxygen too.
• Nothing else. No new fish, no rearranging, no medication courses, no treatments “to help it along”.

Fast or laboured breathing covers the breathing side, gasping at the surface the emergency version of it, and if it persists past a few weeks of good water, gill damage severe enough to need assessment is a real possibility — finding an aquatic vet is the page for that.

What recovery looks like

Useful to know in advance, because two stages of healing are routinely mistaken for the problem getting worse.

The red fades from the outside in, over several days to a fortnight in clean water. Streaks that persist or spread after two weeks of genuinely clean water are pointing at something other than the water.

Damaged fin tissue may go first. A badly burned fin can lose its thin outer edge before it starts regrowing — split rays, ragged margins, sometimes a visible loss of length. This looks like deterioration and is often the tissue that was already dead falling away.

Then the new edge appears, and this is the one that causes alarm: a soft, clear or whitish margin along the fin edge. That is new growth, not fungus. Fungus is fluffy, sits in tufts and starts at a wound; new growth is smooth, even and slowly extends the fin. Treating it as fungus is a common and avoidable mistake.

Full fin regrowth takes weeks to months, longer in a fancy with long fins, and the regrown tissue sometimes comes back a slightly different colour or without the original pattern. That is cosmetic. Torn fins covers the regrowth process in more detail.

When it is something else

If the water tests clean, has been changed regularly, and the fish is clearly unwell, a bacterial infection becomes the more likely explanation. Septicaemia is serious and generally needs antibiotics, which in most countries means a veterinary prescription. Move the fish to a cycled hospital tank if you have one, keep the water immaculate, and get advice quickly. A fish that is gasping at the surface despite clean, well-aerated water may have gill damage that also needs assessment.

Red streaks in a pond

Seasonal, and it catches pond keepers out every year.

The classic case is spring. The fish come through winter with their immune systems slowed, the water warms and they start producing waste again, and the pond’s filter — which has been running cold and largely idle — takes weeks to catch up. Ammonia rises in that gap. The pond through the year page covers the spring restart, which is the underlying problem rather than the fish.

Two differences from a tank. Volume works in your favour, so a pond spike is usually milder and slower than a tank one; it also means a water change is a bigger undertaking, so restarting the filter properly and feeding sparingly matter more than they do indoors. And the streaks are harder to see through pond water from above, so a fish that shows them at all has usually been affected for a while.

Preventing it

Almost every case of red streaks traces back to a tank that cannot process the waste its fish produce. A cycled filter, stocking that suits the tank size, filtration with enough media for the load, and a regular water change routine guided by nitrate readings remove most of the risk.

The conditioner is worth getting right, because this is the one case where the tap is the culprit. Many supplies are treated with chloramine rather than chlorine, and a product that only neutralizes chlorine breaks the bond and leaves the ammonia behind — delivered straight into the tank, at the moment the keeper believes they are doing the fish a favour. Streaking that appears a day after a water change in a tank that otherwise tests clean is very often this. Check the label for chloramine specifically, and if you are unsure what your supplier uses, the water company will say.

The two windows where the risk is highest are the first two months of a new tank, when the filter is still finding its capacity, and the week after any change to the system: a new filter, a deep clean of the old media, a fish added, a heater failing. Neither is a reason to test more anxiously the rest of the year; both are reasons to test daily for a week when something has changed.

Test on a schedule rather than on suspicion. A liquid kit used weekly, and again whenever a fish looks off, will tell you about a problem days before the fins do — and more usefully, a run of weekly readings tells you which way the tank is drifting, which a single reading taken in alarm never can. The water parameter checker will interpret a set.