As long as you're heavy metals, ammonia, and nitrates are kept near zero with zero levels being the best, it should be no problem. Heavy metals clog up gills. You will know if it is a problem if you see the fish 'sneezing'. As for nitrates, it acts like carbon monoxide and the fish will exhibit fast respiration while barely moving or even trying to gulp air and in extreme cases, jump out of the water. I utilize my tanks and pond for all my plant water but my aquaria and pond is well established and if your resivoir is not well established for the fish you risk great harm to both your fish and plants as you will have an ammonia and nitrate spike until the nitrogen cycle is established.