I would stick to the 30 gal filter, and maybe a small internal filter (Fluval 2 plus is nice) unless you can control the flow rate on the bigger filter. If you can't control the flow rate on the larger filter the current will probably be too strong for a 25 gal tank.
You mentioned that you bought "Some" spotted puffers from Pisces (I'm guessing you are in Calgary), so they will probably have explaned that these are Brackish water fish (need salt in the water) but may not have told you that you should use marine salts rather than "Aquarium" salt, due to added minerals in the marine salt that the fish need to stay healthy.
A 25 gal tank is a little small for more than one Green Spotted puffer and you will likely need to upgrade to a 55 gal tank (for a pair, usual recomendation is 30 gal/GSP) in about 1 year or there may be problems with aggression (one will probably kill the other one once they start to fight).
For feeding you will need live or frozen foods, bloodworm, mysis shrimp, brine shrimp when they are small, and muscles, cockles, and other crunchy seafoods when they get bigger (anything from the seafood section at the grocery store, except fish) Although they will readily kill and eat other fish, this is not usually a healthy diet for them and shouldn't be encouraged. You will also want to regularly feed them snails to help keep the tooth growth in check, the staff at the lfs will dig them out of the plant tanks or sump for you.