My son is building fish habitats to place in a local lake. He plans on using concrete to anchor pvc tubes in 5 gallon buckets.

Would any concrete be safe to use? Could he apply a sealer after it cures? Would cinder blocks or concrete have the same effect?

The lake is about 100 acres and 55 feet deep. So there is quite a bit of water to diffuse any chemicals.

Let me know your thoughts. Thank you.
Mugsy