Falling PH, Maybe "old tank syndrome"???

[Processcontrol]

I've had a 110 gallon tank for about 10 years and had it stocked with many various cichlids until about 3 years ago when I lost interest and only kept a few cichlids in it to keep the water cycling. I'm picking the hobby back up and technology has came a long way since, not to mention prices have droped conciderably on fish and supplies. My city tap water comes out around 8 PH but then drops after a few weeks to around 6 or even under.
I replaced all media in my Fluval 403 5 weeks ago and rinsed my 8X3 inch sponge power head filter in removed tank water, also did a 25% water change.
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Added an additional Fluval 404 3 weeks ago.
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My tank completed its mini cycle and I now have 14 cichlids in it.
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What can I do to keep my PH high?
What additives ahould I add? -Please be specific.
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Should I do more water changes and/or more gravel cycling?
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I've been adding Seachem Lake Tangyanikan buffer powder with some success. Should I use more or use something else???

 

[Lonewolfblue]

Since you have cichlids that prefer higher ph, then you may want to mix crushed coral into your substrate, or in your filter, or put a couple pieces of limestone rock in the tank. That will buffer the water up for the cichlids, and you won't need to worry about adjusting the tap water for PWC's. You'll be fine with that.

Just remember when adjusting ph with chemicals, which is not preferred, it must be done slowly. The Crushed Coral or Limestone will slowly disolve and naturally bring the ph up. It will take some time, it's not instant, which is much better for the fish.

Edit:
What is your gh and kh?

 

[Processcontrol]

OK, I got a hardness test kit today.

My GH is very high, over 200. Off the chart

My KH is very low at 20 or less.

What does this mean? Do I need to buffer the water with something? Is it OK for my cichlids? I am still planning on using some crushed coral as soon as I can find some.
Will the coral help my GH and KH?

 

[Lonewolfblue]

Yes, the CC will raise your kh, which in turn raises your ph. But it's a slow, natural process.