We have a old Basement Watchdog Special sump pump system and it is time for a new battery. Basement Watchdog themself no longer sell a flooded replacement and only an AGM (and the product page says its not compatible with the non + unit like ours).

I do know you can get a size 31 marine battery but the its for an older couple that its hard for them to get to and also they kept over filling the old one.

I read that some people just use the Basement Watchdog AGM and it works. Is that a good idea?

Unfortunately they do not just sell a new controller and you need to buy a whole new pump. If that is the only way I guess that's what we will do to have no need to refill anything.

TLDR: AGM battery on older sump pump system designed for a flooded battery or whole new system?