Have you ever had fish as a "pet"? If so look here please!

[NoodleKnight]

I second that, beta fish are really tough fish, you just can't keep more than one per bowl. Sunlight won't hurt the fish, unless its some really radical weather. Sunlight will increase the growth rate of the algae in the tank, though. Of all the creatures said to keep the tank algae free, snails work best, they live fairly long (except when they crawl out of the water, shivel up, and die). The algae eater fish are completely useless unless you have an enourmous tank for it to wander around (its fun to watch them eat), otherwise they die pretty quick.
Goldfish, acutally, don't need the whole complex tank system (a water filter at most, for less tank cleaning), just some clean water and food will keep them happy for a long time (long time as in way past a year). Just be careful changing their water, if its really dirty, and you put the fish in clean water, it dies of shock. Either gradually clean the water, or not let the tank get really dirty in the first place.
Also, at best, try to use some sort of filtered water, a lot of fish die due to contaminants tap water.
Also, about your cat, a mesh cover will prevent the cat from sticking its paws into the tank, you also need to worry about the cat knocking the fish bowl over.
Yes, I have fishes as pets, two beta's. I'm trying to build one of them a macquarium.