Midwest Pond and Koi Society - Q&A #160

Hey...Great site and info and your sense of humor makes it all the better. I'm not sure if you're still answering general questions or not, but if so, here's mine....

I am completing a modest sized pond, with a 40ml rubber liner. I have also lined the inside of the pond (to hide the liner) with these blocks I got from Home Depot, called rumble wall...a nice, large heavy rustic concrete block that looks great as I'm needing more of a formal look than a natural pond scheme.

I was told that since these blocks are formed from concrete that I would have trouble keeping the PH balance because of the limestone in the concrete, and should have used some type of natural rock instead. I won't be putting any fish in there til next spring, but am wondering if it's gonna be a chore to keep this water balanced because of it.

Relining the interior of the pond with natural stone really isn't a good option based on money already spent and labor as well. Is having all these concrete blocks in constant contact with the pond water really a big deal or am I really gonna have to stay on my toes about keeping the water balance where it needs to be.

Also, any comment on these kinds of blocks holding up under the freeze and thaw action during the winters here in E. Tn? After doing all this work I'm wondering if they will end up disentegrating because they can't hold up to the contraction /expansion of cold weather.

Concrete in direct contact with water and fish is generally a bad idea, especially if it is not a specific mix and has not been thoroughly cured, then prepped with muriatic acid and then multiply washed. Any commercial artificial rock not specifically produced for aquatic use has the possibility of sharp edges that have the potential to harm both your liner and your fish and may not have the stability you need.

The lime part of the concrete is not stable in circulating water and will erode over time, depositing sand and sharp-edged gravel on the bottom of of the pond and also exposing it on the bricks themselves for the fish to get scratched up on. Water quality issues aside (and yes, there will be significant problems with your pH) you'll need to watch your water parameters carefully over the next several months, concentrating on pH and alkalinity, and keep a sharp eye on the durability and smoothness of the artificial block.

Best of luck with this. Concrete ponds are common in the hobby, but the mix is a specialized one, and may not be the same as used in your product.