Proper CO2 & Increasing KH

I'd definately raise your kh levels. A lower kh may lead to a bit of instability for your tank's pH.

Personally, I'd prefer adding a small amount of crush coral to a tank, rather than baking soda. It is more of a permanant solution compared to baking soda. I'm not sure about you, but every now and then, I tend to forget to do things... if I had to dose my tank with baking soda, but forgot to, that wouldn't be too pleasant. At least with crush coral, once I get the amount right, I don't need to add any more everytime I do a water change (minus the possibility of replacing it every once in a long while).

I would start off small... perhaps a couple of teaspoons of crush coral in your filters. Give it a few days, then measure your kh, if the level isn't where you want it to be, add a couple more teaspoons. Rinse and repeat. Eventually you will hit the right level.

HTH
-Richer

Agree with Richer on two fronts.
Don't try to up your CO2 until you've addressed the kH issue.
You want to look for a kH of at least 3 - 3.5(4 would be even better) before pushing the CO2.
I also agree on the crushed coral as opposed to baking soda for the same reasons he gives. I do think that I'd add more than a couple of tsp. for a 135 gal. tank. Crushed coral takes what seems like forever to start taking affect, but once it does, it does a good job. It's not a process that will happen overnight though so over dosing is not really a big issue. I would go with 1 - 2 tablespoons and check you kH daily to see the affect. Then adjust from there, looking as I said for between 3 and 4.

BUT FIRST take a sample of your tap water and set it aside overnight. Then run your tests for pH,gH and kH, and see if your readings are any different. Many times allowing the tap water to "gas off" can make a difference in your readings.

Len

I have 6oz of crushed coral in the foot of an old ladies nylon that I add to the tank for a day or 2 after a water change. I find that if I leave it in my tank all the time my kh, gh & ph is low at the beginning of the week and creeps up toward the end of the week.

Instead of just relying on the crushed coral I add 1/8-1/4 tsp of calcium carbonate (purchased at a LFS) and the crushed coral bag for ~2 days after a water change. That makes my kh and ph steady at about 7-7.2 throughout the week.