Nitrites should be zero. Nitrates should not be zero, or near it. Normal nitrate on a cycled tank is between 5 and 20. This could be the problem. You need to buy test kits. These fish are simply too large, and carry too high of a bioload, not to have test kits. In a small volume, levels could fluctuate very quickly. Until these fish reach a pond, you need to be testing at least once a week, if not more. You never state tank size, so I have no idea how long they have. However, if they are 4 inches, you have no longer than one year in which to build a pond, and this is if they're currently housed in a very, very large tank. As for filtration and heating this pond, please take a look at the indoor pond thread. You need to be doing research NOW. These fish won't stop growing, and you don't have much time. Be diligent in your research and planning, and care for these fish the right way. You'll find that it is rewarding, but takes a lot of work. Again, you can look at my gallery for ideas on how to build a pond and the general size of these fish, when half-grown (my RTC is over two feet, and has gotten there in about a year). You have to do right by these guys, and that requires knowing more than you know at this point. It'll happen fast. Please be ready. If you're not, it's not fair to the fish.