I finally found something that I had been looking for to lower the hardness of water for years.

I had read about it, yet could never find one for some reason.

It's called a "Water Softener Pillow" by Aquarium Pharmaceuticals, and it is a resin like substance inclosed in a pillow-like covering.

You place it in your filter, where the water will flow through it and let it work for 48 hours.

Then, this is the cool part, you take it out and
RECHARGE it in a solution of aquarium salt and water for two hours.
Then it is supposed to be fully charged for another 48 hour stint.

The Pillow was $4.49 and then the salt was a few bucks.

I only found it the other day, so I'll let you know how it works.

This may be a less expensive option for Nancy as well.

The water hardness in one of my tanks gets pretty high over the winter months, due to the evaporation rate of the water.  

The tank sits behind my husband's recliner, and when the water level gets down to where the filter starts to make noise, (it interferes with his TV viewing enjoyment)he, GULP, tops it off with tap water.

I don't quite understand how this noise can bother him when he needs the volume on the TV at 20+ to hear it.

Luckily, the biofilter and fish can, and do, survive this treatment.  

Says something about me too, don't cha think? ;o)

Hope this helps, and I'll keep you posted.

The "poor man's RO unit" involved shoving some clean peat moss into a cannister filter and running a 5 gallon bucket of water through it. That stuff is then gradually added to a tank of Rams, rainforet tetras, Apistogramma, Discus or rainforest killies. The peat also adds some veggie material (so TDS may b not drop all that much), acidifies the water and takes about half the mineral out. I suggested that to a buddy whose rams were spawning within days. :)

Don't try that on the guppies though. They would be better served by water a little on the alkaline side.

RO units (see other GL discussions) do have some expenses associated with them. The water should have 99+ % of most things in it taken out. Like the even more pure distilled water, it holds virtually no oxygen. If a fish didn't blow up its gills (after jumping into a barrel of the stuff, as one of my fish did), it would suffocate. :(

My head still swims when I think of how two people keeping discus moved and had an R.O. unit set up in their new home. They put their cichlids in straight R.O. water and killed thousands of dollars worth of neat fish. The only survivors were the fish they were able to net and dump back into the (dirty, but harder) traveling water.

Marine aquarists and aquatic gardeners aound here will use the RO water to remake their tank water using a marine salt mix or something like Seachem's Equilibrium (for freshwater tanks). That way they avoid the abundant phosphates in our water supplies and the subsequent algae explosions.

If your tap water is just mineral rich, you would be well advised to mix it 50-50 or even 40% tap - 60% RO for your guppies. Just as you gradually added water to the bag (or jar) of fish from the shop, decanted and added more water... you might do that with other fishes if taking them into water cut by RO water.

Fish can be moved more quickly into harder water or water with a higher pH. Going softer or lower in pH means one must take more care. Osmotic shock  and gill damage is possible.

RO is also used for baby formula (so the chloramines and ammonia in municipal water don't harm infants). I might add it makes great coffee and orange juice. There isn't enough calcium and other minerals in it to recommend RO water for drinking, though I supposed some tap water could be mixed with it. (That way one can perspire without getting kidney stones. ;) Quoth the non-physician.)

So, except for Scott Lockwood's very important use of topping off to replace evaporation with R.O. water, don't use it "straight up" with fishes.