Hello, I would like to start by saying I know these sizes are way too small for both a betta and a guppy, I am pet sitting this week for what was supposed to be two dogs but I walk in the house and too my dismay there are these poor fish on the counters. The betta is in a stupid EZtankwith no filter or heater and it had a 1/2 film on the top and three guppies in a 2.5 gallon tank. I just was at their house last week so I know they just got the fish and did not cycle the tank. There was so much junk floating around that I got my fishy stuff and did over a 70% water change for the betta adding prime and water dechlor and rinsing off all the debris on the rocks. I then did a 50% water change for the guppies and tested to find the ammonia super dark green. I did three more water changes with prime and dechlor and it is finally under .25. Unfortunately I forgot my gravel vac and there is so much decaying matter on the bottom that I couldn't get the ammonia all the way down. My question is how much of a water change should I be doing for the betta? Also I have tetra safe start can I use that for the betta? I didn't know if you need a filter to use it. After I gravel vac and do another water change on the 2.5 gallon should I put the tetra safe start in? I am unfamiliar how it works. The fishies and I thank you for all the help!