You already got some decent advice regarding cycling the tank.
Just learn the nitrogen cycle. Ammonia > Nitrite > Nitrate. It is broken down by aerobic bacteria. First two are horrible, but the last one is tolerable to many fresh/brackish water fish. Puffers don't handle nitrates as well as most fish, either. They also like warmer water than your fantail. They can both be comfortable at the same pH, though. Bottom line is that is not a great combo.
I used to operate an aquarium shop years ago, took it from 800k a year to 2.4 mil/year gross, in 5 years. Didn't like my share of the profits and moved on. The shop finally, after gaining horrible reviews in spite of the makeover I gave it, closed earlier this year due to a "downturn in the economy." I honestly wish it was still there, I put a lot of work in that place, and I used to buy fish there when I was a kid..