Bottom, middle and top dwelling fish: how to stock a tank by swim level

Most fish spend their time in one part of the water. Some live at the surface, some swim in the open middle, and some stay on the bottom. This is called their swim level. On every species card on this site, the little "swims" bar shows it: top, middle or bottom, and some fish use more than one.

Knowing swim levels helps you build a tank that looks full of life without being overcrowded. It also tells you where each fish eats, what it needs to feel safe, and whether it might jump out. Get it right and every fish has its own space. Get it wrong and you end up with ten bottom fish fighting over the same patch of sand while the top half of the tank sits empty.

How to check it

Look up each fish before you buy it. The swims bar on its species card is the quick answer. Then watch your own tank for a few days, at different times, including just after the lights go out.

Top (surface) fish: spend most of their time just under the surface. They often have a flat back and an upturned mouth for taking food from the top. Examples: hatchetfish, many killifish, some livebearers such as guppies, and gouramis and bettas, which also breathe air at the surface. Seriously Fish notes that marbled hatchetfish spend almost all their time at or near the surface.
Middle (mid-water) fish: swim in the open water, often in schools. Examples: tetras, rasboras, barbs, danios and many rainbowfish. Most of them want open space to swim through.
Bottom fish: live on or near the floor, rocks and wood. Examples: corydoras, plecos, loaches and many small cichlids. Many have a downturned mouth, and some (like plecos) are most active in the dark.

Fish don't follow these rules exactly. Most will swim anywhere at feeding time. Guppies use the top and the middle. Plecos climb the glass. The swim level is where a fish spends most of its time.

When it's a problem

Normal: fish spread through the tank, each kind mostly in its own level. Bottom fish hide by day and come out at dusk.
Unbalanced: almost all the fish use one level. The tank looks empty in one part and busy in another. Fish in the busy level chase each other more.
Crowded floor: bottom fish need floor space, not water. A tall tank can hold plenty of water but have very little floor. Too many bottom fish, or several territorial ones, means constant squabbles over caves and food. Seriously Fish advises keeping zebra plecos as the only bottom dweller in their tank for this reason.
Serious: a fish in the wrong place for its kind. A mid-water fish hanging at the top and gasping usually means low oxygen or poor water (see gasping). A top fish sulking on the bottom, or a bottom fish pacing the glass, can be a sign of stress or illness (see stress and glass surfing).

What to do right now

List your fish by level. Write down which are top, middle and bottom, and how many of each.
Test the water if any fish is out of its usual level, especially if it is breathing fast at the surface. Water problems come first; see ammonia and nitrite.
Watch a feeding. Does food reach the bottom fish, or do the middle and top fish eat it all first?
Check the lid. If you keep top fish, close every gap. Hatchetfish and killifish are known jumpers. See fish jumping out of the tank.
Count caves and hiding spots for the bottom fish. Every territorial bottom fish wants one of its own.

What to do next

Stocking each level

Fill each level a little, not one level a lot. A simple community tank might have a group of top fish, a school of mid-water fish, and a group of corydoras on the bottom. This spreads the fish out and the whole tank looks alive.
Keep schools as schools. Most mid-water and many top fish live in groups. Seriously Fish suggests at least 8 to 10 zebra danios and ideally 10 or more marbled hatchetfish. A few fish of a schooling kind will be nervous and hide.
Count bottom fish by floor area, not litres. A long, wide tank has more floor than a tall one of the same volume. Rock and wood break the floor into separate areas, which helps territorial fish share it.
Don't mix too many territorial bottom fish. Several plecos, or plecos with bottom-living cichlids, will fight over the same caves. Peaceful shoaling bottom fish like corydoras share space much more easily.
Check that the water suits everyone before you pick fish to fill a level. A fish that fits the empty level but needs different temperature or pH is still a bad choice. See water types.

Feeding each level

Each level eats in its own place, so one food rarely feeds the whole tank. See how often and how much to feed for the full guide.

Top fish: floating flakes, small floating pellets and small live or frozen foods.
Middle fish: flakes or slow-sinking micro pellets that drift down through the water.
Bottom fish: sinking pellets, wafers or gel food, dropped where they live. Seriously Fish notes that corydoras take most sinking dried foods, and that zebra plecos are easily outcompeted by more active feeders. Feeding bottom fish just after lights-out, while the others are settling, helps them get their share. Plecos have their own needs; see what do plecos eat.

What bottom fish need

Corydoras: soft sand. They sift the bottom with their mouths and the whiskers (barbels) around them. Seriously Fish recommends fine sand, or smooth rounded gravel kept very clean. Keep them in a group: at least 4 to 6, says Seriously Fish.
Plecos: caves and wood. Most plecos rest in caves or under wood by day. For zebra plecos, Seriously Fish suggests wood with tunnels or crevices, and caves with an entrance only a little wider than the fish. Some plecos need wood to eat as well.
Loaches: most like a soft bottom to dig in and places to squeeze into. Some, like hillstream loaches, need fast, clean, well-oxygenated water instead.
Clean the floor. Bottom fish live in whatever sinks. Vacuum the substrate at water changes and remove uneaten food. See cleaning the tank.

What top fish need

A tight lid. Seriously Fish recommends a tight-fitting cover for hatchetfish because they may jump when startled. Many killifish jump too. Cover gaps around filters, heaters and cables.
Calm water at the surface. Strong filter outflow pushes top fish around. Point it along the glass or use floating plants to calm the surface.
Floating plants: give top fish cover and shade. Hatchetfish tend to gather around patches of floating plants. Don't let them cover the whole surface: leave open water for air-breathers like gouramis and bettas, and for gas exchange.
Air space above the water for gouramis and bettas, which gulp air at the surface. Leave a gap between the water and the lid.

Products that help

A tight-fitting lid or mesh cover: essential for jumpers at the top.
Fine sand or smooth rounded gravel: for corydoras, loaches and other bottom sifters.
Caves: clay, ceramic or slate caves sized for your plecos and other cave dwellers.
Driftwood: shelter for bottom fish and food for some plecos.
Floating plants: shade and cover for surface fish. See the plants section.
Floating flakes, slow-sinking pellets and sinking wafers: one food for each level.
A feeding tube or long tweezers: to put food right where bottom fish live.
Gravel vacuum: keeps the bottom fish's living space clean.

How to prevent problems

Check the swims bar on each species card before you buy, and plan which level each new fish will use.
Choose your bottom fish by the floor size, not the water volume.
Buy schooling fish in proper groups, and add them in batches the filter can handle.
Fit a good lid before you add any top fish.
Feed each level its own food, in its own place.
Give every cave-dwelling fish a cave, plus a spare or two.

Why it matters

In the wild, fish evolved to use a particular part of the river or lake. Their mouths, body shape and behaviour are built for it. In a tank, fish from different levels can share the same water without competing much for space, while fish from the same level compete for the same spots and the same food. Spreading fish across the levels makes a calmer tank, and it means each fish gets fed. A tank planned by level also tends to look better, because every part of it has something to watch.

When to get help

Ask an experienced keeper or your local fish store if a fish keeps leaving its usual level after you have checked the water, if bottom fish keep fighting even with plenty of caves, or if you aren't sure whether a fish will be too big or too pushy for the level you have in mind. If fish are gasping at the surface, treat it as urgent: see gasping.

In the game

In Multiple Tank Syndrome every fish card has a swim level: top, middle or bottom. Two goals use it. The "Community tank" goal needs fish at the top, in the middle and on the bottom of one tank, plus one complete school. The "Clean-up crew" goal in the Starter pack needs 5 different kinds of bottom-dweller in one tank.

With the Plants pack, floating plants such as Amazon frogbit and salvinia calm the surface, so top-swimming fish don't tire from the wrong water flow in that tank. The game doesn't have cards for sand, caves or lids, and it doesn't limit how many bottom fish share a floor. In a real tank, those matter.

Sources

Seriously Fish. Carnegiella strigata (marbled hatchetfish): surface living, jumping, lid, floating plants, group size.
Seriously Fish. Corydoras aeneus (bronze corydoras): fine sand, cover, group size, sinking foods.
Seriously Fish. Hypancistrus zebra (zebra pleco): caves, territory, being outcompeted for food.
Seriously Fish. Danio rerio (zebra danio): an active mid-water schooling fish and its group size.
RSPCA. Fish environment: wide tanks over tall ones, tank size and position.