IME even a 10 gallon is too small. These are very hyperactive fish.:jaw: Feel lucky they are danios, and get a filter. heaters are not really necessary for danios, but here are some basic steps:
1: get a filter.
2: Run the filter in the tank, it should help get rid of the ammonia. (cycling)
NEVER clean 100% of the tank at once. Leave the filter, or save a bit of water to keep the bacteria running.
3:change 50% of the water weekly.
That's all you really need ATM. As soon as possible though, get a 5 or up gallon tank. From my experience, danios will do fine in a group of 4 in a 5 gallon. What's optimal though is a 10 gallon with dense plants.
And a filter will not simply "get rid of" ammonia per se...it may house the bacteria that consume ammonia and produce nitrite as waste.
Anyways...Kaiidera, welcome to AC! I suggest reading this thread to get an idea on what it takes to get a cycled tank. It is probably the most important step you can take in setting up an aquarium.