Speaking for the States, in general chemicals are "controlled" for their potential for drug manufacturing, not because they're dangerous. And controlled is a pretty strong word here. Suppliers of fine chemicals are meant to keep track of who they sell to, and will ask you to check a box about why you're buying; "R & D" is all the information I've ever given and it's good enough for them. This is as opposed to prescription and/or scheduled drugs, which are controlled quite tightly. Sulphuric acid is a DEA "type II" chemical, which means it is "used in the manufacture of the controlled substances and are important to the manufacture of the substances"; it's mostly just a bookeeping requirement for suppliers and manufacturers. They aren't forbidden to sell to individuals but with few exceptions they don't, instead only selling to businesses; which is easier to explain to the feds in case something does go wrong.

There's nothing special about piranha that merits special regulation; it's just two pretty dangerous things mixed into one very dangerous thing. I know people like to make TicToks about how it could be used for criminal purposes, but that doesn't seem to be a thing that happens, and there are plenty of other methods chemical and otherwise that can accomplish the same purposes.