Q. I have two aquariums set up for freshwater tropical fish only and want to addaquatic plants. A problem we have with our tap water is that it is extremely hard to the point where calcium builds up everywhere water evaporates. I have heard that aquatic plants won’t do well in very hard water and tend to develop a “scale.” What is your opinion, and should I be softening the water to grow aquatic plants?
Warren Knight
Southington, Connecticut
A. Aquatic plants that are fussy about the water hardness are few and far between. You can find numerous reports of successful gardeners with the very same aquatic plants in soft or hard conditions. Stick with the tap water. At the Aquatic Gardeners Convention in Houston a couple of years ago, Claus Christensen, Managing Director of Tropica Aquarium Plants, showed some pictures from the area around Bonito, Brazil. Among the photos were some shots of Amazon sword plants growing from a riverbed that was pure calcium carbonate. It does not get much harder than that! The swords were flourishing.
A small benefit of having hard water is that it is often relatively high in carbonates, which will buffer the acidifying quality of added carbon dioxide. If the pH of your tap water is high after it has been out of the tap for a while, you can add carbon dioxide to your aquarium and do not need to add, for example, calcium carbonate to buffer the water to manage the pH drop from the carbon dioxide. That’s one less step in your maintenance routine if you are adding carbon dioxide.
I definitely would not use “softened” water. Water softeners add a lot of sodium to the water while removing magnesium and calcium. That’s all right for bathing but not so good for planted aquariums. Magnesium and calcium are actually trace nutrients for plants. Sodium in small amounts will not hurt aquatic plants, but in large amounts it can.
If you feel compelled to reduce the magnesium and calcium content of the water (and again, I would not), then you would probably be better off adding rain water to the tap water. A more expensive but controllable alternative would be to get a reverse osmosis (RO) filter and add RO water to the tap water.