Plecostomus (Common)
One of the most common seen fish with piranhas from juveniles right up to adults. These are often successful due to the following:
-Grows to good large sizes. 12 inches being common.
-Their body is bulky, protected and heavily armoured. Often too hard for a stray bite to penetrate.
-They are fairly inactive. Hence they do not draw much attention to themselves.
-They will often hold their ground and normally defend themselves. Often using their heads as battering-rams, and their tails as whips.
-Commons are easy to come by and fairly cheap to replace if mishaps do occur down the line. Most people hand large plecos back to the LFS once they outgrow their tank.
They are often seen as algae patrol and a ditcher for aggression. The common pleco keeps the bottom of the tank in movement while the piranhas patrol above.
Tetras (Small and Fast)
These are more common in large adult piranha (especially Serrasalmus species) tanks. And are successful because:
-Their small size means large piranhas will not bother giving chase for something so small.
-Their constant darting speed also means piranhas will not give chase.
-Being shoaling fish means there are plenty of eyes and sensors to track the piranhas.
-Tetras are very common, and very cheap, hence easily replaced if they are occasionally picked off in small numbers from the shoal.
Their fast movements with bright colours often add attraction to the piranha tank. While their small size makes piranhas look much bigger and more king like. They serve as an added attraction, ditcher (to be chased), and help pick up small food pieces from the mess piranhas make while eating.
Pacu
They have been seen to work with Pygocentrus species tanks on occasions. This sometimes works out to be successful because:
-In the wild both pacu and piranha shoal together while young.
-Pacus look very much alike Pygo Piranhas, and are very good at imitating their behaviour to fit in.
-When attacked pacus do not swim in the opposite direction with their back end to the piranha (preferred form of attack for piranhas). Instead they defend themselves head on.
Always worth a shot at trying, however for the space a pacu takes up, it is normally preferred to just purchase more piranhas for the shoal.
Oscars
They have been seen not to only exist with Pygocentrus species but lead the attack on any food the shoal would be interested in. They can work out to be successful because:
-They are very aggressive fish with a bold character.
-Oscars never run away from attacks but face them head on in a full charge.
-In the wild Oscars have been seen defending and fending off shoals of attacking piranhas.
-Oscars are relatively cheap and grow fast, so can easily be replaced if mishaps occur.
Although they may survive with piranhas, they will rarely become part of the shoal.
Peacock Bass
They are one of the lesser experimented with fish, but I have seen the cohabitation with my own eyes. They are sometimes successful because:
-Pbass are again a very aggressive and bold fish that is used-to a structure and hierarchy.
-They are fast when they need to be, very strong swimmers and very aggressive swimmers.
-And has an equally aggressive feeding habit, chasing down food.
- In fact Pbass used to be put into water-holes in South America where piranhas were, to attack or eat the piranhas so farmers cattle would be safe when drinking.
However Peacock bass are arguably too expensive and too pretty to be sacrificed in such experiments where there is no certainty.
Invertebrates (Snails & Shrimps)
If all else fails, snails and shrimp are often helpful in cleaning the tank as they are cheating the tank mate game. This is because:
-The small size means they are barely noticeable.
-The effort to eat one does not gain the piranha much as they are hardly a worth while meal.
-Snails will obviously combat algae while shrimp will tackle the uneaten food particles.
Being that inverts are so cheap and easy to come by, even if a few go missing due to aggressive piranhas looking for a kill, it just means you have offered an alternative food source.