Common problems in small aquaponics systems and how to fix them

Problems in a small aquaponics system tend to trace back to a short list of causes, and the water usually shows them before the fish do. Before changing anything, test ammonia, nitrite, nitrate, and pH, and read the results alongside what you can see. Our water testing guide has the healthy ranges. The symptoms below are the ones small systems run into again and again, each with its likely cause and fix.

Cloudy white water

Milky white cloudiness in a new or recently disturbed system is usually a bloom of free-floating bacteria feeding on excess nutrients. It typically clears on its own within a week or two. Cut back feeding, remove any uneaten food, and skip large water changes, which can restart the bloom. If the cloudiness comes with rising ammonia, deal with the ammonia first.

Green water and algae

Green water means free-floating algae, fed by light and nutrients, and small systems near a window are prone to it. Reduce the light reaching the tank by shading the sides or moving it out of direct sun, and check that a grow light isn't shining straight into the water. Algae on the surface of the grow bed means the top layer of media is staying wet. Lower the flood height or add a layer of dry media on top so the surface dries between floods. A couple of nerite snails will keep the glass clean.

Fish gasping at the surface

Fish hanging at the surface and breathing hard need attention the same day, because the cause is low oxygen or toxic water. Check that the pump is running and the return water is splashing or the air stone is bubbling, then test ammonia and nitrite. If either reads above zero, change 25 to 30% of the water and stop feeding for a day. Warm water holds less oxygen, so a tank that overheats on a sunny afternoon can show this with perfect test results. Bettas and other labyrinth fish breathe at the surface as a matter of course, so judge them by effort and behavior.

Ammonia or nitrite spikes

A sudden reading above zero in an established system usually comes from overfeeding, a dead fish or snail hidden in the media, a pump stoppage that left the grow bed dry for hours, a pH crash, or a medication that harmed the bacteria. Find and remove the source, do a partial water change, and feed lightly for a few days while the bacteria recover. If readings keep returning, the real problem is likely too many fish for the grow bed, which our guide to feeding and stocking addresses.

pH falling or rising

A slow fall in pH is normal, because nitrification produces acid, and the fall speeds up once the water's carbonate buffer is spent. Correct it gradually with a buffer, ideally alternating potassium and calcium sources as our guide to plant nutrient deficiencies describes. pH that rises and won't come down usually points to limestone or shell in the media, or to alkaline tap water. Test the media with the vinegar check in our guide to filtration and grow media, and replace anything that fizzes.

Yellow or stunted plants

Check pH first, because high pH locks out iron. Then look at where the yellowing sits. New leaves yellowing between green veins usually means iron, old leaves yellowing all over means nitrogen, and brown, scorched edges on older leaves point to potassium. Light is the other common culprit, since a windowsill in winter rarely gives leafy plants enough. Our guide to plant nutrient deficiencies covers each one in detail.

Bad smells

A healthy aquaponics system smells like damp earth, or of very little at all. A rotten-egg smell means hydrogen sulfide from waste rotting without oxygen, usually in a grow bed corner that never drains or in sludge on the tank floor. Clear out the pocket and improve the flow there. A sour or fishy smell usually means overfeeding or something dead in the system.

Pests on the plants

Aphids, spider mites, and whiteflies find indoor plants easily. Keep pesticides out of the system, including insecticidal soaps and neem sprays, unless a product is specifically labeled safe for aquaponics. Anything that drips off the leaves reaches the fish, and soaps and many plant-derived insecticides are toxic to them. Knock pests off with a spray of plain water, wipe leaves by hand, remove badly infested leaves, and space plants out so air moves between them.

Pump and flow problems

Flow that slows over a few weeks usually means a clogged intake sponge or roots growing into the pipes. Rinse the sponge in tank water and trim roots back from the return and drain. If you run a bell siphon and it stops cycling, the inflow has usually drifted out of its working range. With too little flow the siphon never starts, and with too much it never breaks. Many desktop systems skip the siphon and run the pump on a timer, which is easier to keep reliable at that size.

Fish that look sick

Clamped fins, white spots, ragged fins, or lethargy usually follow stress from poor water, so test first and fix the water before reaching for medication. Many fish medications harm nitrifying bacteria or plants, and copper-based treatments are toxic to shrimp and snails. If a fish needs treatment, move it to a separate hospital tank so the aquaponics system stays out of it.

If a problem started soon after setup, the cause is often an incomplete cycle, and our guide to cycling a new system explains how to confirm one. For the underlying mechanics of why each fix works, the article on how aquaponics filtration works ties the symptoms back to the filtration chain.