This is my biggest beef concerning canister filters. It seems that people who use them often tend to neglect regular maintenance. Don't get me wrong, cans are great (I have one) but, they do require more frequent cleaning than 7 month intervals, IMO.
WELCOME TO FISHLORE
I seldom purchase new fish, but when I do...
I buy from one trusted source and I do not quarantine.
My source quarantines and medicates all new stock before placing them for sale to the public.
I've never experienced a problem.
I think Dwarf Gouramis are beautiful fish, but they are definitely prone to disease.
Most of them are already infected with DG Disease when you bring them home, IMO.
Sometimes it shows up sooner rather than later.
That's why I don't keep them.
Good luck with yours