First things first and that's reducing the nitrates, do a 50% water change daily until it's down to 0
mg/l or 0ppm nitrates. You won't see the ammonia or nitrites either at that point until it all starts building up when you stop the water changes.
At this point, you can let the nitrates ride up to 20mg/l or 20ppm, and do a 50% water change to cut it in half. Then you'll find the timing to go between 10-20 mg/l or ppm, to schedule a water change. Depending on how much ammonia the fish are producing is how often you have to water change, and you'll see this by the nitrates going up. Above 20mg/l or ppm, it's getting polluted and becoming a bad environment for the fish.
While you're seeing ammonia and nitrites on tests still, you want to keep an eye on those also. If ammonia hits 1ppm, or nitrites and ammonia together hit 2ppm you'll need to do a water change to ensure it's safe.
I won't tell you that you can't do what you are doing, I'm just going to say those fish are going to continue to grow and it's going to get harder and harder to take care of it and keep it healthy. If you are willing to do the work there's room for those fish for another 6 months to a year until you're doing 50% water changes daily because they are putting out 10ppm nitrates or more per day, at that point you'd be doing 2x daily 50% changes. And needing more volume for the fish to swim and more dilution of the nitrates to have less water changing again.
Can it be done? Yes, if you are willing to do the work. If you aren't. Rehoming the fish would be the best for them. When they are 2x their size they are now and it's tight on space, its going to be double what you need to do now to keep it below 20 mg/l or 20ppm,,, which you aren't doing now.
Zero the nitrates with daily 50% water changes, use a
dechlorinator for the new water that you add, and from that point you can
test the water and get a handle on how much water change is needed to keep it safe and below the 20 mg/l or ppm
