low alkalinity/pH in 15 gallon tank

Question
Hi Dave-

I have a few *gulp ok a lot* of questions for you.  Little background--I used to have a 55gallon which i unfortunately had to downsize to a 15 gallon.  I have had the tank set up for a month now and i used all the old water from the 55gallon to start my 15gallon off.  My 55 gallon was always right on the money with regards to water testings and readings (with a pH around 6.5)...now, however, my pH is drastically lower and I am puttin my fish in danger.  As i live in a city, the hardness of the water is also through the roof! I let my water for changes sit out at least 24 hours and add a dechlorinating agent to it as well before i perform the change. Still, my hardness and alkalinity are boadering on toxic levels, PLEASE HELP!
-Also, I have two clown loaches, one is showing drastic sign of "greying out" while the other remains fine.  I know from time to time loaches do this, but could this be due to the horrific conditions my tank is currently in?
-My Silver dollars are also changing...both have now gotten two large black dots on their sides...I have never seen this before and they are a mating pair...please advise.  
-Finally, as i had a 55 gallon tank previously, i had a 55gallon BioWheel filter...this 55gallon filter is the filter that is currently on my 15 gallon tank...could this large filter be the cause of my low readings? Perhaps it is, OVER-filtering the good bacteria? I have a 30gallon whisper filter being delivered to my home tomorrow...
-on that note, what do I do about having one type of filter, and switching it to a completely new/different type of filter?  The whisper filter cannot use the biowheel filters with the good bacteria already on them.  Will it be safe to jut stick the new filter in the tank and have it filter the bacteria as is?? any suggestions??

Thanks for reading all my questions, i really appreaciate it.  Hope to hear back from you soon!

Answer
Hi Corinne:  You can add about 1/2 pound of coral gravel to the tank and the pH will come up.  Add it in 1/4 cup segments at a time though because you do not want the pH to change to rapidly.  Bacteria rarely have an impact on pH; however, a dirty tank will drop its pH quickly...especially if you have lots of live plants that are decaying, etc.  Bog water and peat will drive down pH and both are mostly made up of decaying plant matter.  My concern with the 55 gallon filter is that is may be too strong for your fish to tolerate and this can lead to poor nutrition, fatigue, and disease.  When the new filter comes... I would activate it and remove the other filter. I would also just let the bio wheels float in the tank for a couple of days to aid in regenerating beneficial bacterial colonies within the tank and the new filter.  You can also supplement your beneficial bacteria with products such as cycle and nitromax until your new filter becomes the host for a colony on its own.  The fish are probably going through a natural process... the silver dollars do sometimes get a black dot on their sided... do you know what speicies of silver dollar you have?  The clown loach may just be reacting to the filter and possibly lack of food.  I would keep my eye on him and make sure that he is getting food. You can turn the filter off for about an hour while you feed them... dave