Agree with everything stated thus far. I will give a second to that being way too light filtration for this size fish. I've actually been in this exact same situation. Every time I would feed my rtc my ammonia, nitrates, and nitrites would all spike and fish would go insane and sometimes puke. It was my first fish, and the lfs told me a 125 gal would be good for life. My rtc outgrew it within a year and I tried to make it work. Kept him in there even when he was 24" didn't upgrade as the pet store told me it's ok the fish can bend. He was thrashing water out of the tank every night and breaking the glass heaters, ect. So very similar. I was doing 2x daily 50% water changes to keep my params under control. In the end it was too much, and I couldn't get anyone to take the fish off my hands. Tried every fish store within 100 miles. I went into the air force and had to put him down because no one wanted him. It was a horrible expirience. Wish I had just went ahead and made the pond for him that I was planning, but was trying to rehome him first.
So the moral is do something now rather than wait. It's going to be too hard to rehome him at this point and he needs a bigger tank sooner rather than later. He's already at the size where he would need the capacity of a medium sized sump filter at this point. There is just not enough biocapability in your current filtration. Even if you just had a 10 or 20 gal sump filter you'd be good for another 3 months max. Right now your set up is not safe for your fish. If at all possible go for a rubbermaid container as mentioned above, or go to your local farm, or tractor supply store. I got a 6' dia, 2ft deep cattle trough at one for very cheap! Way cheaper than even the rubber maids. Could last you years in a container like that. They also sold these blue troughs that were like elongated ovals that were 3' wide x 8' long, and 2' deep. Like 300 gals also I believe. Those are sweet and easy to get into a house!!! Might be getting another one myself when I move again!!!
