Hello, I just started a fish tank about 3 days ago. I purchased the tank from a woman who had hurt her back and was no longer able to care for it. It had been functional for over a year apparently and she emptied it about 1 1/2 hours before I came to get it. I got about 12 gallons of the original water, the gravel and decorations were still submerged in the water, and 3 danios, a tetra, and a guppy. I took the whole thing home and had it set up within the hour. I cleaned the tank and decorations (with a vinegar water mix) and rinsed them. I put the water in, and then filled up the rest of it with reverse osmosis water and put about half of the required conditioner for a 25 gallon tank. I figured that because the water has no chemicals in it, it wouldn't need alot of conditioner. So its been three days, I have been watching them carefully. Everything looks good, the danios are very active and happily swimming around and eating good. The Tetra looks good too, breathing is relaxed, not as active but is patrolling. The guppy, however, has been breathing pretty fast since I got her home. I figured once she got in the tank, she'd slow down, but shes still panting and its been three days. Shes eating well, swimming around, not at the top of the tank, shes not struggling when she just hovers somewhere for a spell. I don't know whether to be concerned or not. I am purchasing a test kit tonight, so I can check the official levels tonight, but all the other fish look good. None of them are breathing fast at all. She looks in good shape too, the gills and fins are all good, shes clean and has no spots or anything on her. Shes just breathing really fast. What could be causing this? (Assuming the levels are all good, I will know tonight for sure) Thank you for your time!