What sickness do my fish have?

Okay, so I’m having problems with my fish. I started my new fish tank about 6 weeks ago (it’s a 28 gallon freshwater tank). At the moment I have 3 long finned black skirted tetras, three other type of tetras (I don’t know their name- they’re orange with black and white), 5 small neons, 2 sunset tequilla male guppies and 1 golden gourami. Just in the past two days another gourami and one molly died. In the past (maybe 3 weeks ago) a different molly and 2 other guppies died (I figured those were just stress deaths, since they died not long after moving into my tank).

So now I’m having problems with my fish being sick. Both of the fish that recently died were sick, but I don’t know what of. They were very innactive all of the sudden, and were hiding in corners and going around my heater a lot. Also, they barely ate, and they gulped at the water all day. After three days of this they died. So now a few of my other fish are acting this way, and I don’t know what sickness they have, so I don’t know how to treat them.

I’ve had the usual water balance problems, but now I have nitrites low, ammonia low, and ph is a little high, but not by much. Each week I do a 25% water change, and I have prepared the water in advance. I also have oxygen supplied into their tank, and add aquarium salt on a regular basis.

Oh, in case it makes a difference, I have three live plants and two fake plants. I had a moss ball, but I took it out because it was getting too much algae.

Please help so I can treat my fish!
I add salt because even though it’s freshwater, salt keeps fish healthy. It helps their fins stay healthy, and makes the fish more active. Also, I do make sure there is no chlorine in the new water.

You have new tank syndrome. Your tank hasn’t cycled properly. Your nitrites and ammonia should be zero. The nitrates should be .20 or lower. Get some Ammo loc or an equivalent to reduce the ammonia. Do not any more fish.

Also alage is caused by over feeding. The moss balls actually help in removing alage because they absorb the nutrients in the water that alage needs for life.

salt water fish tanks