Quick answer: bottom feeders do not share one diet. Corydoras and most loaches hunt small invertebrates, so they need a sinking food built on animal protein, such as Repashy Bottom Scratcher. Most plecos, shrimp and snails are grazers, and algae wafers suit them. Cherry shrimp live mainly on the biofilm in the tank, so anything you add is an extra: a small amount every day or two, with nothing left lying on the bottom.
Several of the foods on our shelf were aquarium foods long before isopod keepers picked them up, and some of you buy them for the tank instead of the tub. I have kept tropical fish and shrimp alongside everything else over the years, and the easy mistake is to treat whatever lives at the bottom of a tank as one animal with one appetite. This guide goes through the foods we sell for the bottom of the tank, which animals each one suits, and how much to put in. It is a food guide, not a care sheet for the animals.
Two kinds of bottom feeder
The animals at the bottom of a tank fall into two groups, and the split decides what you feed.
The first group hunts. Corydoras and most loaches work through the substrate for worms, insect larvae and small crustaceans. Gut studies of wild peppered corydoras found worms, crustaceans and other small bottom-dwelling animals, plus some plant matter. In one laboratory study a single bronze corydoras ate between about 41,000 and 76,000 nematode worms in 24 hours. Corydoras are not scavengers living on leftovers, and they are not algae-eaters: they do not have the rasping teeth of a pleco, so they cannot easily scrape algae off a surface.
The second group grazes. Most plecos rasp at surfaces for the film of algae, microbes and fine detritus that grows there, shrimp pick over the same film, and snails graze it too. These are the animals an algae wafer suits.
Corydoras and loaches: a sinking food with invertebrates in it
For the hunters, the food on our shelf closest to that diet is Repashy Bottom Scratcher. It is a gel premix: a powder you stir into boiling water, where it sets into a soft block you can cut up. Repashy describes it as a gel for invertivores of all kinds, meaning animals that eat invertebrates, with all its proteins and fats from invertebrates, led by krill, black soldier fly larvae, mussel and squid meals. It contains no fish meal, and the protein is at least 45%.
Stir one part powder into two or three parts boiling water, measured by volume, and leave it to set at room temperature. Less water gives a firmer gel that holds together longer on the bottom; more gives a softer one. Once made, treat it as fresh food: it keeps up to two weeks in the fridge or six months in the freezer. The product page goes through the method in more detail, including drying the gel into a sheet.
Feed a small cube that gets cleared, not a slab that sits. The gel sinks and holds together where these fish feed, and anything they leave should come out.
Our Pleco Wafers are the other option. They are the highest in protein of our three wafers: herring meal and soya carry it, at 47.4%, with spirulina and other plant matter behind them. They were made as a sinking food for bottom-feeding fish in general, and corydoras and loaches take them.
The specialist advice for corydoras is variety: a good sinking dried food as the base, with small live or frozen foods alongside. What does not suit them is a plant-heavy algae wafer on its own. Corydoras will mouth one, but algae is not a food they seek out, so do not rely on a pleco's leftovers to feed them.
Plecos: wafers at lights-out
"Pleco" covers a whole family of catfish and more than one diet. Most of the family feeds on fine detritus and algae, and the common bristlenose is a grazer with brush-like jaws built for scraping. Some plecos are meat-eaters, the zebra pleco among them, so check what yours is before you choose its food.
For the grazers we stock three wafers, the Pleco Wafers among them. Spirulina Algae Wafers are the plant-heavy one, at 34% protein. Oak and Algae Wafers sit in the middle at 38.3%. All three sink and hold their shape in water, which matters for a fish that feeds slowly and mostly after dark.
Spirulina, a green ingredient in all three, is a blue-green alga, strictly a cyanobacterium, and it is rich in protein. A review published by the UN's Food and Agriculture Organisation puts that protein at 55 to 70% by dry weight, which is why it turns up in so many foods for grazing fish.
Add the wafer late in the evening. Many plecos feed at night, and a wafer that goes in at lights-out is still there when they come out for it. Feed what the tank clears overnight and lift out what is left in the morning.
One thing to clear up, because people ask: bogwood, or driftwood. Plecos rasp at wood, and what they get from it is the film of algae, microbes and detritus on the surface. Studies of the wood-eating species found that the wood itself passes through largely undigested, and fish kept on wood alone lost weight. So wood in a tank is a surface to graze, not a food, and the oak bark in our Oak and Algae Wafers is not there to do a pleco's digesting for it.
Shrimp tanks: biofilm first, food second
Cherry shrimp feed on biofilm, the thin living layer of algae, bacteria and other tiny organisms that coats every surface in an established tank. In one 2020 study, cherry shrimp grew, matured and bred through a full life cycle with biofilm as their only food.
That makes anything from a packet an extra. Most advice comes down to a small amount every day or two, and less in a mature, lightly stocked tank where the shrimp graze between meals. A common rule among shrimp keepers is to give only what is gone within two or three hours. Either way, nothing should be left to sit, because leftover food fouls the water and feeds whatever else is in the tank, snails first.
From our shelf, a piece of any of the three wafers suits a shrimp tank. Break a small piece off, since a whole wafer is a lot of food for a colony of shrimp, and you can always add more tomorrow. Shrimp are omnivores, so a small cube of Bottom Scratcher gel suits them too.
Freeze-dried daphnia works too, once it has been soaked so that it sinks. Our guide to what daphnia is and how to feed it covers the method.
Snails in the same tank graze the wafers alongside the shrimp. Freshwater snails are known to take up calcium for their shells from the water itself, so I would not choose a food for its calcium just for the snails.
Shrimp keepers are wary of copper, and with reason: copper dosed into the water, as in some fish medications and snail treatments, is toxic to invertebrates. The trace copper on a food label is a different matter: it is there as a nutrient, foods made for shrimp carry it too, and shrimp use copper in their blood. Our Pleco Wafers list it in their mineral premix, and Repashy lists a copper compound in Bottom Scratcher. I have not found a feeding study on cherry shrimp, so I will not call any amount proven safe. The danger on record is copper in the water, not copper on an ingredients list.
How much, and when to take it out
- Gel: small pieces that get cleared, with the made-up gel kept in the fridge or freezer between feeds.
- Wafers: in at lights-out for plecos and other night feeders. Feed what the tank clears overnight and lift out the rest in the morning. In a small shrimp tank, a piece that is gone within two or three hours is better still.
- Daphnia: a soaked pinch as an extra, not a staple.
- Flakes: a pinch the fish clear in two or three minutes, with anything left netted out. Our tropical fish flakes feed the middle and top of the tank; what drifts down is a bonus for the bottom, not a meal.
If you are unsure, feed less. A tank that is slightly underfed clears its food; an overfed one fouls its own water.
You will find the other aquarium foods we stock on foods for reptiles, turtles and fish.
Frequently asked questions
What do cherry shrimp eat?
Mostly biofilm: the layer of algae, bacteria and tiny organisms on the surfaces of an established tank. Sinking foods such as a piece of algae wafer, a little gel food or soaked daphnia are extras on top.
How often should you feed cherry shrimp?
Every day or two, in small amounts, and less in a mature, lightly stocked tank. Advice differs on the detail, but it all agrees that leftovers should come out.
Can shrimp eat algae wafers?
Yes. Feed a broken piece, because a whole wafer is a lot for a shrimp colony, and lift out whatever they leave.
Can cherry shrimp eat fish flakes?
They will pick up flakes that reach the bottom, but flakes are made for fish feeding higher in the water, so I would not rely on them to feed shrimp. A piece of sinking wafer is easier to portion and to take out again.
What do corydoras eat?
Small invertebrates: worms, insect larvae and crustaceans, with a little plant matter. In a tank they need a sinking food of their own with animal protein in it, varied with small live or frozen foods.
Do corydoras eat algae wafers?
They will mouth them, but corydoras hunt small animals and do not seek out algae, so algae wafers alone will not feed them. A sinking food based on invertebrate or fish protein suits them better.
Do plecos need driftwood?
Not as food, on the evidence. Plecos rasp at wood for the film that grows on it, and studies of the wood-eating species found the wood itself is largely undigested. I have not found a study that tests plecos kept with and without wood, so treat it as a grazing surface and feed them as well.
How do you make Repashy Bottom Scratcher?
Use one part powder to two or three parts boiling water, measured by volume. Stir it in, then leave the gel to set at room temperature. It keeps for up to a fortnight in the fridge, or six months frozen.
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