There are some good suggestions in this thread. There are also some misinformation here. First of, 8 dKH is perfectly fine, if your test kit is accurate. Recommended alkalinity is between 7-11 dKH. A lighting period of 6 hours using 400W DE is perfectly fine. That's 6 hours of intensive light period. You don't get more than that hours of intensive light period in the tropic seas. I am using 250W for 6 hours and get good growth (For some SPS, they are growing like weed).
For OP, are your corals happy? How is their polyp extension? What kind of corals do you have? Maybe they are just slow growers. Try to find some fast growers, like pink birdsnest or green birdsnest or some digitata and see if they grow at all in your tank.
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