Plattys are dying

Question
Good morning,
I have been having difficulty with my 30 gallon freshwater aquarium.  Last weekend I did a 25% water change.  I changed the carbon filters, and added new water w/ a "Stress Coat" water dechlorinator.  The tank only contains 2 guppies, 2 plattys (or "Mickey Mouse" fish - as we call them,) and one Chinese Algae eater (not sure of the real name.) 24 hours after the H2O change, I tested the water and the ph and alkilinity were low.  I added ph up according to the instruction.  The plattys started acting unusual.  2 nights ago one of the plattys started just sitting on the bottom and not eating.  Then last night, the other one was "swimming" on its side - like it was going to die.  I didn't expect either to live through the night!  This morning, both were still alive - the one still sitting on the bottom (and with one 1 spot on its tail - maybe the beginning of ich?), and the other swimming normally.  The ph and alkilinity are still too low.  I've owned aquariums for only 4-5 years, so I'm far from an expert.  Any help is appreciated.
Thank you.

Answer
Hi Christine,
I never use anything to alter the ph due to the fact that it is usually hard on the fish. Any sudden increase in ph by a chemical is harder on them than them just gradually adjusting to a ph that isn't ideal for them.  Do your fish have any problems when you just do a partial water change and don't do any adjustment to the ph?  Also, if your water sits in your aquarium for several weeks without a partial water change, the ph will drop and the fish will adjust to that and then when you do a water change the ph will be somewhat higher and the fish can have a really hard time adjusting to it.  Be sure, if you are not doing so already, to do frequent partial water changes--at least once a week.That will at least keep a ph that is a little more stable.  Platies and Guppies are usually very hardy little fish and can adjust to water conditions that might not be the ideal for them but close. I don't know how low your ph really is but it could be that if they don't show any stress from a regular partial water change without adding any ph up that they can adjust to your water without any problems. If you add the ph up to your water if will cause them  to get sick from it more so than just seeing if they adjust to the lower ph that your water has. It is really hard on them even with a small sudden rise in ph so be careful. Try doing a water change and  use Amquel Plus or Prime water conditioner in your replacement water and be sure it is the same temperature as the tank water. I have had really good luck using these water conditioners since they not only remove chlorine and chloramine but also help with the ammonia, nitrites and nitrates in your tank. Of course don't ever depend on a water conditioner to take the place of frequent water changes. Just do the water change and see if there is any improvement in your fish. You may not even need to worry about the ph.
I am sending you a website that has some information on ph that I thought might be helpful. Best of luck!
Hope this helps,
Karen
http://freshaquarium.about.com/cs/waterchemsitry/a/waterph.htm