I have to recommend a fishless cycle too. And a liquid test kit. Yes it sounds complicated, but it's not when you make an effort to get your head around it; my 8 yr old son got it after a few explanations. And guys on these forums will help every day if needed. I completed my first fishless cycle using neat ammonia solution a couple of months back, and it was actually fun! And once you've done it, not only will your fish be healthier and happier, but you'll understand what it means to keep their environment healthy. It will save you frustration, time and money in the long run. You say that your older tank has happy fish - forgive me if it sounds harsh, but their dorsal fins are down, suggesting that they are not as happy as maybe you'd like to think. Probably too high ammonia/nitrite/nitrate in the tank!