1/4" or better polycarbonate panels cut to size would be my choice, but relatively expensive. The corrugated thinner greenhouse panels would work and are cheaper but don't really have a finished look imo if that matters to you.
If this is just for practicality, I just drape and cinch a clear vinyl tarp over my stock tank which works very well to keep fish in and insulates rather well.
If this is just for practicality, I just drape and cinch a clear vinyl tarp over my stock tank which works very well to keep fish in and insulates rather well.
