PH Levels not stablizing.

Question
We have a 29 gallon tank that we've had for over 3 years.  We currently have 1 adult platy, and 8 platy babies, and one pleco that is about 4 1/2" long.  About 5 months ago, our water softener went on the blitz.  Before that, the water was the perfect ph, all the fish were fine, then things drastically changed.  We had black mollies, white mollies, and platies, the pleco, and a panda cory.  Most all of the fish died.  The platy we currently have is one that was pregnant when we got her to add to our fish...this was before the water softener broke.  All of our original fish died except for the pleco.  

My question...Why, when I do a water change, does the ph drop again to match the 6.4 in the tank?  The water in the tap is actually 7.4, but then it plummets to the 6.4 the next day.  I want desperately to bring the ph level back up so that we can add to our tank again (most specifically the panda cory).

I want to do this naturally.  I don't want chemicals in my tank if at all possible.

We will be getting a new water softener in the near future, but as of right now, we just have regular city tap water.  The water softener seemed to help the fish so much when we moved about a 2 years ago, the just seemed happier and less stressed each time we changed their water, then when we had regular tap water at our other house.

We do have live plants, I have checked and they are all fine.  No rotting leaves, no rotten roots, etc.  They are growing nicely, and so that doesn't seem to be what is causing the PH drop.  I change my carbon filter (with a bio wheel) every month.  Even though the ph is stable, it is so close to dangerous, I worry about adding even one new fish.  I really want to raise the ph to get me to have peace of mind.  

I noticed someone spoke of crushed coral.  How does that work again?  How long would I put it in?  Just have never heard of that, but willing to try just about anything.  Thank you,

Aaron

Answer
Hi Marsi/Aaron
 
Try testing your tap water.  Get a glass of water and test it.  Let it sit out at least 24 hours, and re-test the glass of water.  That should be what's happening in your tank.  If it's different, then you know that it's something in your tank that's causing it to drop and we need to figure out what it is causing it.  If it's adjusting that way from the tap, then you'll need to buffer the water before adding it to the tank.
Is there any driftwood in the tank?  If so, that'll descrease the ph.  Another thing that could cause it to drop is insufficient surface agitation.  That keeps the CO2 in the water, which can lower it.  
In reality, 6.4 is fine for the fish you have.  Most of the fish we buy at the store are tank bred or farm raised as opposed to being wild caught, so they can tolerate ph from 6.0-8.0 just fine and there's no need to adjust it.  The thing that worries me though, is the sudden drastic drop-which can stress/kill the fish.  

Crushed coral should work.  It contains calcium carbonate, which helps to buffer the water and increase the ph levels by making the water harder.  You can buy it at the pet store, it's in a big bag similar to gravel, usually it's used for marine tanks. I would recommend putting it in a mesh filter media bag(also at the pet store, usually by the filter media section), and putting that into your filter if it'll fit.  If it won't, I know it doesn't look attractive, but hang it near the filter or set it on the bottom of the tank-whatever.  The reason for putting it in a bag, they're little small sea shells-hence the name crushed coral, and if you scatter it on the bottom of the tank or use it as a substrate, it tends to trap debris and eventually will raise your nitrate levels in the tank.  I would just leave it in the tank permanently, or until your water straightens out with the ph-assuming it's the tap water that's the problem.  Limestone rocks work as well.  Basically if you find a rock, pour some vinegar on it.  If it foams or bubbles, it contains calcium carbonate and will most likely increase your ph in the tank.  

And good for you for not wanting to dump chemicals in :)  Pet stores and such are too quick to push those chemicals and medications.  Most of the time it's just putting a band aid on the problem, instead of addressing the source of the problem.  Here's a good link I have that can better explain about ph, gh, and kh:

http://faq.thekrib.com/begin-chem.html

http://www.chelonia.org/Articles/waterchemistry.htm

Hope that helps and good luck!

Christy