Some fish aren't growing

[gilpi]

From my personal experience, water changes are key to growth, there are other factors, food and tank size. I have literally seen fish grow faster the more water I change. Also you might be stuck with fish where their growth was stunted prior to you getting them.

 

[Lynnzer]

Where did you get the idea that prawns contain thiaminase and shrimp do no not? For that matter I was of the understanding that shrimp and prawn are virtually synomonous, the different words referring to size, but different cultures use them differently. British (and commonwealth) english uses "shrimp" for small and "prawn" for large. American english uses "shrimp" as a more catchall.

https://www.practicalfishkeeping.co...ed-to-know-about-predator-fish-and-thiaminase says that thiaminase is found in cyprinid fish, mussels, and crustaceans

Both shrimp and prawns (which are common name classifications not scientific; both are decapods as are crabs, lobsters and crawfish) are crustaceans. In fact, a fish fed on nothing but brine shrimp and/or daphnia might also have problems.

Thiaminase will probably be destroyed by cooking, but it also probably destroyed all the thiamin in the food before cooking.

Regardless, shrimp and prawns should not be the main diet. I think the main issue about thiaminase is that feeder goldfish are a really bad choice of live food.

I have fed cooked prawn to many of my fish. Not on a regular basis but pretty frequently.
One of the main problems with feeding fish is that most of the stuff that manages to hit the tank bottom stays there and sours the water. When is enough food given? No-one can really give a clear indication except for stop feeding them when they don't show further interest.
I used to put a cooked prawn on a wire and hung it into the tank. when the fish were fully fed I lifted the remains out.
I also used to feed cooked mussel to my firemouths.