I took a peek down my overflow with a durso standpipe a few months ago (some algae was clogging up the standpipe), and holy moly - I also had a similar aiptasia garden going on.
I'm considering removing the standpipe and turning off the return pump for a weekend to see if drying them out for 48 hours will work to kill them. However, I've yet to do this and haven't had any major outbreak in the DT- this garden has probably been there for many, many months.
Currently, I don't think I have any aiptasia in the main tank that I'm aware of. When I do see one pop up, I aiptasia X it and then thoroughly cover with super glue and epoxy. This system seems to be working out well as long as the hole gets covered thoroughly with super glue/epoxy.
But knowing that garden is there definitely gives me the heebie jeebies that a bad outbreak could happen again. On the other hand, I don't want to try and chemically eradicate them as all that'll do is anger them off and guarantee more pop up in the DT . Hence, I might just to dry them out at some point.