It's a good idea to get floats in a couple different seasons and see where YOUR count stands.
Doing "group" floats is considered an accepted method.. multiple small samples combined into one test.
There are some mail in services that will test much cheaper! Floats run $25 at my vet, which is...
Hi there, welcome to BYC! :frow
You can treat with Corid as it won't hurt.. but that's very unlikely your issue unless there are newly added birds to your flock that might be carrying a strain that yours haven't developed immunity to or vice versa.. and your vet doesn't sound super avian wise...