dying shaving brushes

[burblerboy]

I have a three gal. Dwarf Seahorse tank set up with 2 air/sponge filters, 12w power comp. conv. bulb(full spectrum 6500k), 1 mangrove, some caulerpa mex.(feathery stuff), halimedia(small type) and the shaving brush as well as my cleanup crew and 2 pair of dwarf seahoses. My temp stays between 74-76F, sg:1.021, PH:8.3, ammon.:0, nitrite:0, nitrate:.02. I keep the lights on for about 10 hours/day, also have red/blue LED night lighting. I do approx 50% water changes once a week to maintain water quality for dwarfs. My problem is that I can't keep the shaving brushes alive (this is my third one from two sources) It will be ok for a couple weeks, nice bright green then just start falling apart. All the bristles fall off then the main stalk folds in half and breaks off or falls down. I also add a little Purple Up once a week to help supplement calcium and iodine. Can anyone tell me what I may be doing wrong? Or any other ideas, thank you in advance.

[malsavant]

How deep is your sand and how long have you had it?

[graveyardworm]

My guess is lighting shaving brush prefers higher levels of light, or perhaps your getting poor specimens. What are your other parameters like Ca Alk and Mg. I'm guessing that with your water change schedule they're fine, but it would good to know.

[burblerboy]

It's about 2" deep aragonite based sand bed. The tank has been set up for almost 18 months, just got the dwarf seahorses in May this year(2 months ago)

[burblerboy]

I'm not sure about the other parameters, I havn't tested beyond normal tank quality.