some info on the serrasalmus rhombeus from the web:
1) Although these species do not require any extensive care, a commitment is still required. If you have a 100 liter tank and see a 10-12 cm black rhom or gold you will probably need to upgrade your tank to a minimum of 200 liters with at least 400 GPH of filtration. I've raised mine in 245, 265 and 340 liter tanks. Highly consider a 55 liter feeder tank with 100 GPH of filtration. There is nothing worse for your ammonia level than the introduction of a few feeders into the tank. Not only when it rises with the initial introduction of eight to ten feeders, but when your black rhom eats two and kills the rest your water quality has been compromised. I purchase eight medium size feeders, ones that can be devoured in two bites and then after eight days I do a 40% water change to the feeder tank and let the new feeders sit in the tank for two to three days.Besides healthy feeders, only feed the piranha RAW meats like chicken and small portions of lean beef. Farm raised earthworms are a nice treat, as well as seafood. Never hot dogs, pork, cooked meats or insects found in your garden. The cooked meats produce grease that give the species problems and outdoor worms could contain those lovely garden chemicals used extensively.
2) If you are looking for a species to entertain guests while having a party, please do not select a rhom or gibbus - get an Oscar. In all my years of piranha keeping, rarely did any of my single species immediately devour feeders in front of me, let alone 8 people standing around the tank. Piranhas are purchased with this entertainment in mind, but beware it could be some time before the piranha eats in front of you. I've seen people purchase piranhas with this in mind only to find themselves returning the fish after a month because it is a boring species. BE PATIENT, let the Rhom adjust to its new environment. Keep this species in tank in a corner alone - they spook easy. If your piranha spooks and inflicts a gash upon himself, turn the light out and wait about ten minutes, then carefully turn the light on to assess the damage. Normally medication is not required as these fish have unbelievable healing powers. This is also the reason that I only keep Amazon swords in the tank and not rocks and driftwood and I use a background that covers three sides of the tank.
3) Water changes are important, as these predators create a high level of ammonia just by their feeding regimen. I currently have a 23 cm rhom in a 245 L tank (soon to be move to a 340) and I do 75 L water change every 10-14 days. If your water quality begins to suffer, the first sign is usually that the fish's eye begins to cloud. This is a sign to change water and consider a carbon change in the filter.
4) If you need to medicate a piranha, never use a wide spectrum medicine (i.e., medicate the entire tank). I will not feed mine for three days, after which I will cut a pocket in a piece of raw chicken and then put a small amount of medicine in the food. I found this to be far and away more effective than using one teaspoon for every 10 L of medicine and you don't have to remove your carbon cartridge.
Thanks for your time to read my experiences. Just be patient with your piranhas and they will reward you with years of enjoyment.
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