I know I wish January would roll around because I need a check coming then. (The wolf was at our door, so we roasted it.) If I'm short of a certain kind of fish food, I'm absolutely sure I might even try veggie flakes with my carnivores if other foods were in short supply. ;) After vacations herbivores may devour bite sized tank mates, guppies may serious forage in the hair algae and plants may be a bit tattered in that tank with the one big predator, so the fish themselves will improvise if need be.

However in time your guppies are passing a lot of those starches through their systems. I'm sure bacterial colonies will develop to use the carbs but then you might have a cloudy effect from them. That may mean more partial water changes (arrrrrrgh!). So you and your fish are best served using them for more no more than the short term. :)

You had a zillion question last year and it really kept us hopping. That was a lot of fun. Glad to hear that you are still keeping guppies and mollies.

If your friends went through goldfish pretty fast, you might discreetly ask them if their tank had cycled. I think that Angelhologram's fish-less cycling article is still available here.

Goldfish are poop machines. They also bulk a lot more than most aquarium fish, which we commonly begin with. Larger aquariums should probably be used with even young ones and a few good sized adults would appreciate something like a 55-gallon aquarium, if they get over that crucial early period and are on the way to a life expectancy of 25.

Lastly, and I realize that this is more for those looking on because you probably know this, it is popular to try one's hand with those inexpensive feeder goldfish, but they tend to be floating museums of parasites. I would get a commercial broad spectrum anti-parasite treatment and treat them during that first quarantine period. Otherwise there might not be any other period when they are kept.