Low pH

It probably is directly related to the PH of the water you are using. Tap water varies greatly. The driftwood can also lower pH by what it contains, from what I've read. For the fish that you have I wouldn't be concerned about the ph unless it keeps dropping, or you have problems with the fish, most of what you have listed is quite adjustable, and the neons actually like the ph around 6. Chemicals rarely work, and can cause just as many issues as they might solve. The other option is expensive, and that is to get and RO (reverse osmosis) system for your tanks, as that water typically has an even 7.0 pH.

The first thing to check is the pH and carbonate hardness or buffering capacity of your tapwater. If your local tapwater is acidic, you've found the problem. If its buffering capacity is low, humic acids and other byproducts of the breakdown of driftwood and fish waste will rather quickly lower its pH. Chemical buffers such as the Proper pH brand sold in most aquarium catalogs are benign and there's no reason to not "like" them. If you're serious about the aquarium hobby you're going to have to learn a bit of chemistry and to get used to adding various salts to your water to make it more hospitable to the species you're trying to keep. Addition of a few oyster or mussel shells will also help to raise your pH or increase carbonate hardness, but the effect is somewhat hard to control. Another thing to keep in mind is a problem that, in the hobby, we call "old tank syndrome", in which an aquarium has accumulated so much mulm and the water has become so laden with organic acids that it's difficult to bring up the pH and buffers are quickly exhausted. The solution is to clean filter media, remove mulm from your sand or gravel, and begin frequent water changes until your aquarium reaches the desired pH. If you were keeping fish from the African Rift Lakes, most of North America, or the Congo River, pH 6 would be cause for concern. I don't know what a cleaner is, but most of the fish you listed would find such conditions quite comfortable. Angels and neons come from waters in the Amazon basin rich in organic acids, and Gouramis come frome stagnant and equally acidic asian backwaters.

Are the fish fine? None are dying off? Are you trying to breed any of them? If no to all of those, then just leave it alone. A stable ph that stays the same is much better for the fish then one that fluctuates by adding chemicals and such. Stay away from the ph up or ph down, it'll just cause problems when the ph crashes on you and kills off your fish. If you want, remove the driftwood, otherwise just leave it. Most fish are tank bred, so they can adapt to pretty much any ph. The only ones that would have a problem with a ph being wrong would be a wild caught fish. And, from what you've listed, I doubt those were wild caught. If it's not broke, then don't fix it :)

Maybe you have to plant or you can take some salt to the water

Chemicals are about all you can do. Its a chemical inbalance in the water you need to work on. There are simple kits you can buy at the pet store or walmart that'll fix your problem. Some live plants wouldn't hurt either.

How often do you change you water? Having the driftwood on that tank lowers the PH, plus the fact that the PH of the water coming from the tap. I usually use crushed corals to bring up the PH of the water.