Sounds like bacteria bloom. Generally not harmful and will clear itself up in a few days to a week. Just keep watching the ammonia and nitrites and nitrates and doing water changes as needed.
Keep your nitrAtes, the end results of the cycle, below 40ppm and your fish will be happy. Lots of people recommend below 20ppm. I generally test my water before changing it and do the math to figure the amount to change enough to bring it down to less than 10ppm and it will build to around 40ppm by the time I do it again.
A note on salt: Livebearers like your swordtail tend to appreciate a very low concentration of "marine salt" to a specific gravity of around 1.001~1.003. The danios and gouramis don't really need it or benifit from it, but aren't harmed by it. And if you get a pleco or other type of catfish they hate any amount of salt.
Note that I don't say "Aquarium" salt, that stuff is just NaCL (common table salt) and is pretty useless in an aquarium unless treating ich, and then there are better ways. Marine salt is a mix of various minerals and trace elements that add buffers to the water that make it slightly harder and stabilize the PH along with a bit of NaCL. A SMALL amount of marine salt goes a long way towards helping fish tolerate higher nitrate levels than they can without it. Takes a lot of research for the details, but in a nutshell it promotes a good slime coat and adjusts the osmotic pressure of the water a little bit making it harder for certain chemicals to get through the gills. Something like 1 tablespoon of marine salt per 10 gallons.
IF you decide to add salt, you should always disolve it completly OUTSIDE the tank. If a fish sees a salt crystal it will think of it as food and if it eats it can burn itself on it. Either by having a gallon bucket with water standing to be used at the next water change, or cheat and disolve it in a glass and just gradually pour it in as you change water if you use a python.