WHOA! Forget acidity, thats a minor issue and unlikely to be the cause. Stress and ammonia are alot more likely.
You've got an uncycled tank. Fish eat food, and what comes out the other end is ammonia. Ammonia is harmful to fish in anything other than tiny concentrations. Bacteria build up in the filter that break down ammonia and convert it into nitrite. Nitrite is also harmful to fish, and another bacteria builds up which breaks it into nitrate - this isn't too bad (though higher nitrate levels will affect sensitive fish.)
That bacteria takes a long time to build up naturally. I've got to get some sleep - I've got to be up for 5 for work so thats as much detail as I can go into about the nitrogen cycle.
Once again, I refer you to my previous point - don't trust your fish shop - especially on this. Trust me regarding the following and you've got the best possible chance of keeping your fish alive - if you don't theres a fair chance you'll lose some fish:
1. You need a liquid based test kit as soon as you can afford one - you're looking at about £20. The API Freshwater Master Test Kit is highly recommended. Buy one and do the tests. They are cheaper off the internet.
2. Until you get a test kit, do daily 30-40% water changes. Use tap water and dechlorinator - theres lots of different types, anything that says Tap Safe or Safeguard or something similar will give you an idea - go into your LFS and ask for a dechlorinator and they'll probably stock 3 different brands of the stuff. This will lower the amount of ammonia in the water at any given moment. When you get a test it, test daily - if theres any ammonia at all, do a big water change.
With the first water change make sure any excess food on the bottom gets removed - decomposing food also breaks down into ammonia.
3. Cut the feeding right the way back. Fish can survive off next to no food - trust me on this. Feed them a pinch (the amount they'll eat in 1-2 minutes) every other day. The less food in=the less ammonia out.
Those points should see you through. Avoid the desire to tinker with the tank, as messing about with it stresses the fish out - and they're already stressed from moving about. Leave them be and most importantly do points 2 and 3.
Someone else can fill in the gaps I've left, I've really got to go to bed now - I got 4 hours sleep last night, worked today and am looking at 5ish hours sleep tonight. Hope this helps.
Just to reiterate: Water changes are good. Always. As long as you're using dechlor and you don't change more than 40% you'll be fine - and twice daily 40% changes is fine. I wouldn't recommend more than that though as changing 40% of the water does involve a temp change, and more than that may have adverse effects. So spaced out big water changes are good.
Fish love water changes, having flicked through the above thats not entirely clear. And in your case - they need them to prevent ammonia levels building up. Ammonia will kill your fish.
