Look up thiaminase on google, you will find articles like this one,
ghost shrimp, mollies, guppies) will be needed to initiate feeding and many individuals may never accept anything but live fare. You should attempt to train your Rhinopias to take pieces of shrimp, squid and fish off of a feeding stick. One thing you should avoid is feeding your scorpionfish a diet consisting only of feeder goldfish. Raw goldfish flesh contains thiaminase, an enzyme that causes the breakdown of thiamin. If you feed your scorpionfish a diet that consists only of goldfish, they may become thiamin deficient, which can result in feeding cessation, clamped fins and nervous in coordination. If you have to feed them live food, gut pack mollies, guppies and/or ghost shrimp with a nutritious food (e.g., Cyclop-eeze®) before you feed them to your Rhinopias (some individual may be reluctant to feed on live ghost shrimp).
and this,
should be obvious from the above that thiamin is an essential component of energy metablosim. Without sufficient thiamin, animals have impired pyruvate utilization, causing increased plasma pyruvate levels and a shortage of cellular ATP. Thiamin deficient animals also have below normal transketolase activity, and therefore a good test of thiamin status is to test the amount of erythrocyte transketolase activity an animal exhibits. Because thiamin is so essential to energy utilization, general signs of thiamin deficiency include weight loss, impaired feed utilization, and weakness. More defieciency signs are detailed in the pages under Signs/Symptoms of Thiamin Deficiency.
Some thiamin is also used to form thiamin triphosphate which is thought to have a function in brain cell viability, though its exact role is still unclear
and,
Thiaminase destroys Thiamine (Vitamin B1).
12.) Regular intake of substantial amounts of food containing thiaminase could introduce enough thiaminase into the gut to break down the thiamine in food & render an animal thiamine-deficient.
13.) Some fish contain thiaminase (Type I, not II) & some dont.
and,
Thiaminase destroys Thiamine (Vitamin B1).
rosey reds are almost as bad.
Here is a link,
http://www.merckvetmanual.com/mvm/index.jsp?cfile=htm/bc/170904.htm
Even though all the above is true it takes time and the diet needs to contain a high % of goldfish to develope thiamin deficet. The build up of copper can also be a problem in characins like piranha.
My Elong will ONLY eat goldfish, I tried to feeder guppies and roseys, and even neon tetras... From what I gather, anything small enough for him to just, "swallow" he will not bother. I've had 10 guppies in the tank with him for along ass time. hell the guppies are breeding! 
