Buffering for a higher pH?

I'm having a bit of a problem in keeping the pH of my tank water from
dropping too low. Here's the setup:

20 gallon long tank, using vermiculite/loam substrate covered by Aquarium
gravel, WDF 3000 wet-dry power filter, DIY CO2, and 2-20W FL tubes, one a
Coralife Trochromatic, on for about 11 hrs.

The tank is planted with Hairgrass (dying off, it seems, ever since I gave
it a trim), a little Cabomba (I think the fish are eating this, though), a
small plant, which I don't think is an aquatic, as it's not taking to the
tank at all, and some Hygrophilia, which is taking over the tank! The Hygro
is just loving the substrate, putting out really goot roots, generating
lots of leaves, and just looking good. I use Tetra FloraPride, and Hilena
Crypto-Dunger tablets to fertilize.

The occupants:

1 Otocinclus affinis
2 False Siamensis
2 Opaline Gouramis
4 Corydoras julli
5 Danios (4 zebra, 1 pearl)
6 Tiger Barbs
8 or 9 ghost shrimp (can't ever see all of them at once)
1 Ramshorn snail (about 3/8 inch-a stowaway on a plant)
1 other snail (haven't ID'd this one yet-another stowaway)

Yeah, I know it's on the overstocked side, but between the plants and the
biofilter, the water chemistry has been quite good. I try to keep to bi
weekly 25% water changes. The fish all look healthy, there is no aparrent
disease, coloration of all is very good (esp. the barbs).

Water chemistry:
Ammonia: Just showed a trace last night. Probably due to the stocking
level, which would explain why the Hygro is doing so well!
Nitrite: zero
Nitrate: about 30 ppm
pH: 6.0
General hardness: about 8

I don't have a KH test kit, but I will get one this weekend.

The tap water I use has a pH of 7.0, 0 Ammonia, no chloramine. I treat it
with Stress-Kote before adding it to the tank.

I just changed 25% of the water last weekend.

I know the fish seem happy, and nothing seems 'wrong', but I wonder if the
low pH isn't stressing the fish. I'd like to see the pH keep to about 6.5
in this tank. Hopefully, by the end of the summer I'll have another tank or
two set up, and I'll be able to ease the congestion in this one.

So, what methods are available to me to a) get the pH up to about 6.5, and
b) maintain it at this level. From what I've gathered, I need to increase
the buffering capacity of the water in order to get and keep the pH higher
than it wants to be. I know the CO2 affects the pH, but I'm not ready to
spend $3-400 on a 'high-tech' CO2 system.

Thanks for any help you can offer.

Larry West