Concrete in fish tank

it should be fine if its uncolored. just put them in some water and test the PH after a day or so other than that you should be fine. just make sure you dont knock the bottom off the tank
 
Concrete can be detrimental to your tank. I would at least do the above test in a separate tank over a two week period to determine if it is going to give you any wild pH swings. You could always paint it with aquarium safe epoxy or paint however.
 
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.
 
If concrete is poorly made (which is easy to do) then it will leech all sorts of nasty salts and stuff into the tank.
 
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