My tank is a 7 lirte tank i change the water every two weeks (i would do it every week but i have a great filter and plants) im getting it tested next week so i will tell you, no but i have it next to the sun so the water gets warm naturlly, my goldfish is just a fantail.
OK, thanks. Well, your tank is small and here's the deal. The smaller the tank, the sooner you have to change the water. Even with plants and a great filter, 2 weeks is way to long. You
might get away with that on a much larger tank say, 50/75 gallons but not on a little one. It's simply a matter of space. Ummm, one drop of waste in an ocean of water v/s one drop in a glass of water. Hope that helps. I'd change the water at least once a week, maybe more depending on your readings. (suggest you get a water test kit. I just git one for $25) As far as the sun goes, sunshine is pretty much the enemy of fish tanks. Promotes algae growth and can easily over heat the water. It's a small tank so it doesn't tank much. Now about your fish. As others have pointed out, goldfish and zebras shouldn't really be together. A goldfish is a Pond Fish and a Zebra is a Freshwater Fish. I know, they both live in freshwater but water temperatures are very different between the two. A zebra's water should be kept at about 78 degrees where a goldie, any goldfish prefers it much colder. Say in the upper sixties. Also, both your fish will quickly outgrow your tank. Some people say a fish will grow to the size of a tank so don't worry about it.
Not true. Here's how it works. The outside of the fish will grow to the size of the tank and stop but his insides keep growing to normal size. This of course eventually leads to the death of the fish. Sad story but these are the facts.
Anyway, easy solution. Get a bigger tank and heater and stuff! Hey, you're having fun right? Sure! So just keep your eye open for the stuff you need. New or used, doesn't matter. I see them on Craig's list all the time at great prices. Sometimes for free. But beware of MTS. (multiple tank syndrome)
Just kidding. Hang in there. The fun is just beginning.