How many mulities?

They're pretty hard to sex so the bigger the group you can buy at a time (I'd say 6-8 is enough) the better the chances are that you'll end up with a couple that will breed.

I can't say for sure what you limit is. I kept mine in a 55 gallon and only had to remove a couple that were causing problems (likely males). The setup lasted only 1.5-2 years so I never had a huge population explosion. My julies probably picked off some fry also. As I said it's a judgement call as they'll reproduce pretty constantly. Just make sure you have A LOT of shells lining the bottom of the tank (2-3 per fish at the least) and go from there.
 
If your going to do a 40gal i would start out with 10 do you have a place where you are getting them i have a lot of them for sell at 3 dollars a piece i also have Brevis Kavala at 6 bucks a piece 1 2/2" plus i am in Ky Louisville area..
 
How about substrate? I was thinking about using 40lbs of the eco-complete cichlid sand, is that enough? or are there better options

Thanks for all the help youve given me so far

 
Eco-complete cichlid sand is one of the very best options if you can afford its price. It will help to keep the high pH and hardness your multies like the most and need to thrive. IMO, a substrate bed 1 1/2 to 2 inches deep is enough. I think 40lbs is enough for that.
 
IME substrate isn't all that important for shellies, at least to them. They'd rather dig down to the bottom of the tank. I ended up with huge mounds of sand on either side of the shell pit. But its fun to watch them dig. The males will pick up sand in their mouths, scurry to the outskirts of the pit, and dump.

You shouldn't need much sand. 1.5" is enough really as shells are far more important.

Unless your water is very soft, you shouldn't need the eco complete either. I'd personally save money by going with pool filter or play sand. Even with softer water I'd likely go the crushed coral route. Your choice though!
 
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