Do I need to treat my main tank

[jedi940]

Ok this may be a dumb question but Do I need to just treat my main display tank.

Background:
I have had my tank up and running for about a month and a half. I was doing good until I bought some guppies from the new pet smart that opened. I was thinking new store = clean tanks. however, every guppy that I I have purchased there has died. The jury is still out on the final two but out of 6 guppies, 4 have died. After the most recent death, i noticed that the tail of the fish was missing and deduced that a possible tail rot fungus was responsible. Since then, I have purchased a quarantine tank and lost another guppy (not from pet smart - infact the first fish i bought) and a molly to mouth rot and cotton mouth) two days later, my gourami OVERNIGHT came down with fin and tail rot, mouth rot, and cloudy eyes. that gourami is now in the quarantine tank being treated for fungus and will hopefully pull through. Now I think my male guppy might be starting to get tail rot as there is a small split in his tail fin that I don't think was there yesterday.

Soooo, back to my original question, do I just need to suck it up and treat my 55gal display tank instead of continuing to remove sick fish and treat them in the quarantine tank?

tank specs
55 gal
running for a month and a half, cycled with nitrates at 10 ppm
stock:
2 mollies (1 has died)
3 guppies (2 have died)
6 neon tetra
3 yellow tetra
snail
2 gourami (1 being treated for 3 different fungi)
penguin 330 filter
tetra whisper 60 filter

[Bigge3415]

What are your water parameters and how did you cycle your tank?

[jedi940]

I cycled the tank with fish in it. The fish were 3 neon tetra and 2 guppies. I have tested regularly and my ammonia is 0, nitrites are 0 and nitrates are around 10. ph is 7.2-7.4.

This is from the api testing kit. two days ago I took back one of my dead fish to the fish store for a refund and they require a water sample. Their results confirmed the above. Additionally, they tested hardness and alkalinity. Hardness was 250ppm and alkalinity was 80ppm according to their test strip. Water temperature is around 75 degrees. I do not have a heater yet but have one on order and hope to have it within the week and will turn up the temperature to around 78-79 to minimize fluctuations.

Also, since the original post, i have since moved the other gourami to the QT as he was showing what I believe is a hole just in front of his eye. I have yet to move the guppy as I a not convinced that he is sick yet.