The new tank was more of a challenge than the original one. That's a given. Part of the problem was the fact that it's a smaller tank, part of the problem was probably hubris - the first tank went well, so why shouldn't the second one? Well...it didn't go so well. I probably overstocked the tank in a big way - one adult platy and four babies was more than I should have added. I also made the mistake of using the bacterial starter colony that came with the tank. Since none of them have expiration dates or information about how it was handled, there's no way of telling whether an inoculum is likely to be viable or not, but common sense says that bottles costing a few dollars have a higher turnover rate than a complete aquarium kit.
Anyway, after a few days the tank stank of ammonia - smelled like a litter box. I did a 50% water change, but after two days the smell was worse than it had been initially. So I got some Amquel + (Sodium hydroxymethanesulfonate) which is supposed to react with the NH3 and NO2- and convert them to products that are harmless to the fish but bioavailable to the bacteria. Since then, the smell hasn't returned, though I should really test the NO2- levels.