Unless it's old it will effect ph. Most people will use a fish safe epoxy paint to seal the cement. I've seen a lot of casket holders converted into bait tanks.
If for some reason(cost,availability etc.) you must use cement as a fish tank you'll need to acid treat it first. I've read that they apply muriatic acid twice and rinse the same amount of times. Apparently the term used for the process is deflocculates, gets rid of calcium.