I think 12 danios is a little over doing it, particularly if you aren't into them. Platys are pretty hardy as is a betta, but I would personally stick with like two or three. We take a 10 vacation and I use those tablets and haven't had any problems. The only thing I can add is doesn't say how many fish it will feed, I think it just says how many days its good for, so seeing as how fish can go for a week or so with no food, I just drop in one for five days in the larger tanks and the 10 g one get a 3 day one.