How old is he now? I hope he's not still in a bowl - red ears get huge!
We're talking a shell diameter of over a foot, and they grow very fast when healthy! They also live for a very long time (30 years+), and it can take years for them to show health problems when kept incorrectly - by which time it's too late to do anything to save them.
I know this from dealing with many abused red-eared sliders - so many people have no clue how to look after a turtle and pet stores are usually useless for information - most just want to sell things.
Stop feeding him fish - and get him on a diet of 1/2 turtle pellets (from a pet store, these contain the necessary vitamins and proteins) and 1/2 fresh greens - not iceberg lettuce, spinach, curly kale - anything that's suitable for tortoises should be suitable for turtles, you will need to check on google before feeding.
Turtles are very specialised animals to keep and require very large aquariums, with very large filters and expensive UVB light fitting and heat bulbs. If you cannot commit to owning and maintaining a massive aquarium (I would recommend a minimum of 2 foot wide and 4 foot long) with specialist reptile lighting and over-sized filtration - then return the animal to the wild where it belongs
.
This page is a good read: http
/www.tortoisetrust.org/articles/res.html .