I am new to the hobby and have scoured a good chunk of the aquarium co-op videos but I am stumped. I have a 20 gallon long, plants, driftwood (has clear fungus, but was told this was not harmful) gravel, few store bought decor, and a sponge filter. I started with about 10 neon tetras, 1 rubber lip pleco, 1 hill stream loach, and 1 nerite snail a little over 4 weeks ago. Immediately a handful of the tetras died off. I did not thinking anything about it and assumed shock or bad brood from the store. The pleco died after a week, it never did seem to eat and only hid. Over the course of the following weeks a couple more tetras died. I started noticing what looked like ripped fins. I assumed fin rot as there is no observed aggression, and since I did not quarantine I went ahead and treated the tank with melafix, paracleanse, and super Ike. I just finished the regiment and another tetra died, and the fins on the last two look more torn with white tips starting. On top of that the snail has been staying a the top of the tank out of the water since just before I started treatment which sounded like water quality but perimeters have been consistent and tested daily with API master kit.
ph: 7.2-7.6, ammonia, nitrite and nitrate: 0, kh: 54 (3 dkh), gh: 160 (9 dgh)
Please advise as I have no clue how to proceed at this point. Thanks in advance.