A snake owner for less than a week and I can offer some advice .

From what I've read flounder is OK.

There's a good article here:
Thiaminase

Although there are also articles online that suggest that flounder can contain thiaminase, the balance seems to be that it doesn't. From the reading I've done salmon and tilapia are definitely safe fish, but that doesn't help you get rid of the flounder in your freezer.