After folks, management is convinced of the effectiveness of DIY CO2,
maybe they'll get wise and buy a gas tank system.

This will do two very good things for the store.
If they sell reef of planted tank it will give them experience and the
ability to sell such mechandise. It will allow them to spend far less
mainteance time of both Reefs and Planted tanks.

The same CO2 gas tank and regulator can drive an array of tanks, both
for Reefs, eg, Kalk reactors to maintain alkalinity and Ca levels in
coral tanks and planted CO2 FW tanks.
All you do is add another needle valve and a brass 1/8" T which run
10.91$ + 1$ for the T etc from www.clippard.com and the store can sell
these for about 20-25$ each retail.
The Regulators for 51.95 from Kegworks.com.
Gas tanks locally.
Silicone airline for the lines.
A reactor is all that's left and of course peripheral things like pH
test kits etc.
A single system can supply an entire store which CO2, I did a store
that now has 20 tanks with CO2. 12 FW and 8 SW kalk systems.
A single 50lb tank.

They fill it once every two years or so. Cost 30$.

It's much more precise than maze DIY systems qnd better long term CO2
supply for folks. You can do CO2 to a high level with DIY, but
............we tend to slack off as people at some point. The gas
really is 100% worth every penny.

The LFS will get the use and the sales out the system. But show them
the usefulness of CO2 first.

You can also have the LFS box up generic fert's in LFS labeled
containers, eg KNO3, K2SO4, KH2PO4 etc.
You can get all that at grass turf places and ag stores in 50lb sacs.
They run about 25$ and 50 units x 5$ each for K2SO4 and KNO3 = 250$ of
profit. For the KH2PO4, 2 oz for 5$ = 2000$ for 25$.
They can make their own traces using CMS plantex etc also.
Or you can sell them the stuff also.

Good luck.
Regards,
Tom Barr