I have had bare bottom and up to 2" of sand on several tanks. I wouldn't say one is better than the other but I have always had the best results with a coarse,2-5mm, sand bed around 1". It has probably been due to a lot of factors but I think if you have sand you need to gravel vacuum it every few months minimum. In general I prefer the sand bed look and to me it adds some stability. Nutrient levels I strive for are detectable nitrate, that means any color(0.2) to 5-10ppm nitrate with the salifert test kit. And a minimum of 0.03 phosphate. I turn on the gfo reactor when I get to 0.1 or 0.05 if I'm having algae issues, and off at .02-3. Without enough flow BB or not won't help, you need to get the detritus up off the floor for export with BB or it will just collect in a corner, and higher flow will keep it from settling into the sand. Personally I like more smaller power heads to fewer larger ones. More directions of flow reduces dead spots and smaller power heads reduces the extra high flow areas that corals tissue tears and can't survive.
For po4 removal change gfo every week, but only use around .25-.5 ml a gallon and push a lot of flow through it, more than usual. Put the output tube in the filter sock to remove the dust generated from the grinding of high flow. This should get your Po4 down the fastest with gfo, for lanthanum chloride a little, like 1ml for your 300g, twice a day is good and finish off at around 0.10 maybe a bit lower.
As far as Hanna checkers go, I prefer to go by the phosphate test kit. I have gotten readings of 0.00 with the phosphate and 40ppb with the phosphor, around 0.12 phosphate when converted. This is about where my tank stays with a lot of macro growth in the sump pulling out the po4 I presume. They read different kinds of phosphorus that react differently with corals. I know it's kind of weird and I don't try to figure it out anymore... I just try to use the same test kits and run at a consistent level and the po4 readings are more stable, while phosphor seems to rise with water top off over time. It seems to be in my top off water and not get used up by my macro algae. Also I have found if you do a lot of testing, like I did for a while on a dozen tanks a week, you want to buy your reagents in bulk, like 4-8 packs at a time, so they are all from the same batch. I have seen inconsistencies from different lot numbers on the same tank at the same time. Multiple tests from the same lot number read consistent but there was a big difference from one lot to the other.
Also have to put in the MP-60s and move the MP-40s.
Then again, the phosphates in my live rock holding tank were over .65! I'm going to dose some lanthanum chloride to bring it down in the near term, but I think its mainly just due to me stirring everything up...
