Tank thriving / guppies dying?

Question
HI there,

I have had a 10 gallon tank for almost a year.  Since the beginning I have had 2 cory catfish, 4 neon tetras, and 2 african dwarf frogs - I lost one frog recently out of nowhere other then that - those are all still alive, happy and thriving.  I also had a couple guppies and mollies over the first couple months but none lasted. I had horrible snail infestation issues that after 6 months of picking them out and scrubbing jelly egg sacks they are finally gone. I also have one nice size apple snail now....

Then I got a group of 3 guppies, they all did great for a couple months until I went away (think my pet sitter overfed a bit) and skipped 2 weeks of water changes and then discovered I had a bad case of red pest. Now, the one (yellow) guppy with the red pest signs, red spot, red streaks in tail has lasted the longest (until this morning) :(  the other two died when with no symptoms before I even noticed any streaks/symptoms. I treated the tank twice (two 4 days cycles with 3 day rest in between)  with antibiotics recommended by a local aquatics store. The yellow guppy with streaks seemed better for the last month, streaks disappeared, red spot still there but faded, tail fin definitely a little thinner but not getting worse - I waited a month from treating to getting 2 new guppy friends - my guppy was getting so lonely he was hanging with the tetras. Anyway - 2 new guppies and one died in 24 hours. Then my yellow guy started acting really sick, laying still at the bottom of the tank or on my rock decoration, today he is gone, and now the other new guppy is having trouble staying horizontal. What the heck is going on?   All of the tetras, corys, frog seem totally fine.   

All my levels seem fine. I think my tank is a little on the acidic side (helped keep snails at bay) but not too much as my apple snail is fine. I've been keeping it clean....  I have 2 live plants and a big rock, nothing else. No heater, but I haven't needed it.  I do get algae blooms kinda often, but I have a sunny house and don't like the fake paper background (I tried).    

I cannot figure out why everything is fine but my guppies keep dying.  Are guppies more sensitive? Am i just getting crappy ones? My best guess is overfeeding - but wouldn't everything suffer and my tank be dirty? I would like a couple top feeders to balance my tank - what is a more hearty option?

Huge thanks for the help. Sorry for the long post, was trying to make sure I gave you all the needed info! -Deanna

Answer
Deanna,

It sounds like a water issue. I would say add salt to the water but your tetra's will not like it. All live bearing fish tend to do well is slightly brackish water. If your water s acidic, that could be the problem. If you are getting the guppy's from the same store, their supplier might be giving them sick fish. I would try getting them from another store. You can also just make a tank just for guppy's If you do that make sure you have the water they way they need it. When you get the new fish, do you put them in your hospital tank for a minimum of 2 weeks before adding them to the main aquarium? This is a very important step in fish keeping. This helps keep sick fish out of the tank and keeping the other fish healthier. Make sure you are doing weekly water changes of 25%. This will also keep things cleaner at all times.