The solution would be to get a bigger tank. You can try whatever method, they may work they might not work. But at the end of the day, you need a larger tank to cope with both the size of your fishes and the bioload they produce. I'll not discuss on the size of the fish since you are just pure stubborn to admit that the RTC isn't gonna last a lifetime in your tank and i'm sure many people around here have already told you about it.
So lets talk about bioload. RTC are messy eaters, produce lots of waste. Sure you can upgrade your filtration, get a diatom filter to get all the bacteria in the bloom out of the water. It may work for awhile. But as the RTC grows, so does the bioload. Even an FX5 isn't capable of handling the bioload of a full grown RTC. And you are gonna get this bacteria bloom again. So what are you going to do? upgrade your filtration? Thats just dumb, with that $ I would rather invest it in a bigger tank. So the real solution is this
"OMFG GET A BIGGER TANK"