Lots of problems in one...

[.223NATO]

Ok so, I have few problems with my tank and any input would be greatly appreciated!
To start... I have a 100g tank that I started at the beginning of February, with 125 pounds of live rock (I started with 100lbs of dead rock and I seeded it with 25 pounds of live rock) running two canister filters and a small pump filter that was built into the hood. I only have three fourty watt bulbs. Originally I only planned on having a fish only predator tank but I soon changed my mind after it was to late and decided to have a reef tank insted.
Phosphorus: 0
Nitrates: 0
Nitrites: 0
ammonia:0
calcium: 440
KH: 220
PH:7.9
Silicates: 1

To start I have always had a problem with my PH. It hhas always been low. I have used Reef Buffer numerous times to bring it up but after a few hours or over night it drops back down to 7.8 or 7.9 . I have never had a problem with Nitrates or anything else. I recently bought a fish that I didn't quarintine and it introduced Ich into my tank and since then I lost a powder blue (that it came with) and a regal tang. Now 2 of my clowns and my foxface also have ich so I set up a quarintine tank yesterday that I am going to treat them in and then after that use to introduce new fish so I don't suffer the same losses.
I have also had a hair algae problem in my tank that I am assuming is due to the fact of how low my PH and the silicates in my tank. I haden't changed the carbon filters in my tank since I started it until a few days ago. So I put new carbon in and I bought two Opti-carb carbon packages and put those in to help.
My only other problem has been that I can't seem to get coralline to grow in my tank. There are a few very small spots that have some new coralline besides the stuff that was already on my live rock when I bought it. I just recently switched from Sea Chem's Calcium builder to Purple-up in an attempt to try something different.
Any thoughts on any of these problems wou'd be much appreciated! Am I on the right track here or can you guys see anything else you think I haven't addressed here?

[cantfindnemo]

How long has your tank been set up?

[.223NATO]

since the beginning of february, so about four months now...

[bertoni]

http://www.reefkeeping.com/issues/2004-09/rhf/index.php

That article discusses low pH problems. I think you should try the aeration test described in it. Most pH problems come down to high CO2 or measuring problems, so another thought is to doublecheck your pH test result.

Reef Buffer and similar products really aren't a good approach to low pH issues. They have a temporary effect on pH, but seldom solve the problem, and lead to high alkalinity levels, which can be a problem. The alkalinity level given is already a bit high.