I've researched this a lot over the years. Mainly because I hate paying for something I can make. Also the possibilities of what you can make are endless. Every piece could be different. With all of that aside there are tons and tons of threads where people talk about different mixes one can use. Then there are tons and tons of threads about how concrete can leach chemicals into the water over time. Not sure what chemicals. Aside from the possible leaching. People have horrible problems getting there PH under control. The average ph of concrete is 12.8-13.5 something like that. I've read threads where people have been trying to get there ph below 12 for 6+ months doing 25% water changes every other day.
All I'm saying is do your research on this before you get to crazy. I'm sure a couple pieces wouldn't be a problem but when you fill a tank with it is where you might start running into issues. I gave up on it my self just doesn't seem worth the issues.
