Those feeder fish, get between 12-18 inches and have lifespans of 30 plus years when properly cared for. The rule is 20 gallons for the first fish and an additional 10 gallons per additional goldfish. Goldfish should never ever be put into bowls. It is cruel, would yall keep a great dane puppy under the cupboard of the bathroom sink? Sure the dog would survive but it wont be healthy or happy. Same with the fish. What happens when you keep a goldfish in a small container, the outside seems to have stopped growing but the insides continue to grow, eventually killing the fish.
Frequent partial water changes are a must. For 3 little comets in a 30 i would do 25-30 percent partial water changes weekly, where the gravel is vacuumed. Goldfish are extremely dirty which is one of 4 reasons why goldfish should only be housed with goldfish. The second reason is different dietary needs. They should have different brands of goldfish food as well as real plants and veggies. Such as anacharis, cucumber and zucchini slices, and fresh or frozen shelled green peas.
The third reason being temperature differences. Goldfish have primitive gills, requiring a lot of dissolved oxygen in the water. Cold water holds more dissolved oxygen then warm water does. Mollies, guppies, platies, cory cats, ottos should be kept in heated tanks. Mollies are actually brackish fish and require salt.
The fourth reason being, that the fish could be carrying diseases that they themselves aren't affected by but other fish can be and vice versa.
I wouldn't mix commons and comets with slow moving fat fancy goldfish. The longer bodied goldfish are faster, and can be more aggressive. They will out compete the slow guys for food.
Careful with plecos. Commons get 2 ft and have been known to eat the slime coat on the goldfish. Plecos themselves are dirty and they cannot survive on what grows in the tank. They need supplementation with algae wafers, cucumber and zucchini slices, as well as driftwood for fiber. What they clean up in the aquarium, they make a whole nother mess with their crap. If you are strict on tank maintenance you will not have a lot of algae growth, certainly not enough for an algae eater, plus if you aren't, the poor water quality will quickly kill the pleco. I find they are sensitive to high nitrates.
