I don't know about blocks, but concrete has lime in it which will leach into your water. A while back (like 5-10yrs) my dad and I built a couple of goldfish ponds in his yard as water features. We laid down poly0ethylene, and then covered that with concrete. The day after adding water, it turned green. That was from the leaching of the lime from the concrete. Long story short, after about 2-3 months, it stabilized and cleared up. So, I believe you may have some prblems in the beginning. You might try putting it in a bucket or steel tub of water to see what effects it has there before testing it with your fish. They may have a bad reaction...and that would be bad.