Fish tank with no filter?

For cold water fish specifically goldfish you need a filter that provides double filtration to the tank (so twice the size of the tank 20 gallon tank, 40 gallon filter). Goldfish are extremely messy and a filter helps clean the tank. It also holds the bacteria that eats the ammonia and helps the stability of the tank. Without a filter you should be doing a couple water changes a week if you have a properly sized tank and are not over stocked. If not you will not have fish very long.

No Clean Fish Tank

Danios and catfish do best in warm water. And if you are talking about comet goldfish, then they must have a filter as they are really messy fish.

All the fish you mentioned do badly in tanks without filters. Goldfish are exceptionally messy, if only you had the chance to look into the poop infestation in my filter when I open it up for cleaning every week -.-"

The only fish a person who knows nothing about fish should every try without a filter is a betta.

Even still, you need to do partial water changes every couple weeks. (Changing about 20% of the water is good.)

Don't get the fish until you get the filter. Take the money you were gonna use to buy fish and get a filter instead. It needs to be set up to cycle 4+ weeks before you can add any fish anyways...

U might be able to but, it depends on how big the tank is and what kind if fish, bc gold fish are like mutts they usually don't need to much speacial treatment, and if Ur cleaning it so mug Ur gunna want to make sure Ur watching the CO2 levels in Ur tap water, cuz it can kill Ur fish

What kind of fish?

betta, gourami, paradise fish, other air breathers would do ok in still water but would do better with a little filtration.

you're going to want something to disturb the surface and let gas exchange. at the very least an airstone.

you will kill the fish without a filter. plain and simple.

you need a filter!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!