How many mulities?

I only keep 1" of sand substrate for Shellies they will excavate your sand bed..
 
Multis are a great fish. I used to have a nice colony in a 20long. The males are generally larger than the females and have a longer looking body. If you buy a larger number of the smaller sized and a few larger ones, it's a good chance you'll get a good male/female ratio.

Great fish ! I've been thinking of starting up another colony myself. I sold the last batch off to make room for a reef.
 
You will be fine the substrate will buffer it to maintain your properties.. Most Tang's can hold at 9.0 PH and over 400ppm Hardness..
 
The chart in my AP KH/GH testing kit only reads to 12 drops of test solution and it requires about 15 drops so i dont know the exact KH reading Ill retest right now and make sure thats right its been about 3 weeks since i first checked it and i havent checked it since.
 
I only keep 1" of sand substrate for Shellies they will excavate your sand bed..

Well, after all, that's the depth of the sand bed in my multies tank.
 
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