why are my corals alwayes turning brown

Honestly, it could be a bunch of factors. Lighting, nutrition, trace elements, etc. I've read a bunch of anecdotal information about doing this or that, using this bulb, swapping to T5's, using Zeovit, dosing different additives, etc. You name it, there's someone who's had success with it.

So, I'd first make sure that you have good water parameters. High nitrates are bad, but near zero nitrates could also be bad. How about phosphate levels? Do you have wild temperature, PH, or salinity swings? On the same note, how are alkalinity, calcium, and magnesium levels? Have you tried bigger or more frequent water changes? It sounds like you do plenty. In the case of zoanthids, are they getting enough iodine (anecdotal info there)?

Next, I'd look at lighting. Are the corals getting enough usable light? 20k bulbs usually have lower PAR than a decent 14k or a 10k bulb. You might want to try changing to a higher kelvin bulb- if so, try a Phoenix bulb first, and then maybe an AB or Megachrome 10k. Brown corals could mean that they're compensating for low light by allowing the zooxanthellae production to increase, slowly turning brown. Or, they're getting too much light and the same algae is going crazy in their cells. I doubt it's the latter with 20k bulbs, but I could always be wrong.

I thought that color pigmentation, etc. was a BIG topic and that Dana Riddle was really getting into this. As I remember, coloration of corals has also been linked to what depth they were growing at. I seem to remember that red corals usually came from depth, and so on.

The above post is just my opinion.

Here's a side shot of before the algae takeover. Coral coloration went downhill from there. It deteriorated until Anthony Calfo said that I needed more fish to pollute the tank, making more nitrates, and increasing coloration. Although I think there may have been some truth to that, my tank had algal issues and I was hesitant on adding more fish. I miss that tank...