Here's what I use. 15 gallon, I think I got from US Plastics. I have a float inside on some fishing line with a weight outside so I can see the level. I put 3 gallons in it once a week. It's on a covered porch I can see through a garage Window.
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I've thought about doing something like that. I've shopped US plastics in the past for a tank for my methanol injection system.
At the moment I'm really thinking I'm just going to use a 5 gallon bucket with a lid that I drill a hole into for the tubing. I think I've decided to calibrate my dosing based off time instead not watching the quantity dispensed. I have verified that it dispenses approximately half a gallon an hour, so I should be able to figure on how many days I get to a bucket full.
I've started with one hour a day, and will use my testing to see if that maintains the approximately 8-10 ppm I want to keep, overshoots that, or undershoots. I can always disable it for a day or so to let the levels drop, or manually dose as I have been directly from a jug into the pool.
As to preventing someone from installing a Frog bac pac, I don't see why I can't find a short length of chain with a link large enough to slip around my injection quill, and loop it around the bottom of the frog cartridge and lock back to itself. Sort of like a bike chain.
It would be easily defeatable by unscrewing the injection quill, but I don't think it has to be impenetrable. It just has to make someone say "Hmm, maybe I shouldn't do that." I'll also fashion a warning label to put on the lid.