When we look at a tank, so often we're flying on instruments - without an instrument rating. :(

Your water changes are good. They are removing a lot of "stuff" dissolved in the water beyond just the "hardness" minerals.

If the swordtail has been there a couple of days and there was a dramatic increase in either hardness or TDS, his gills could have been hurt (some) by the osmotic change. He should be able to heal if the stress then and now is not too great.

Do you or your husband know somebody either with an R.O. unit for home consumption or who works in a lab where they wink at people taking a little demineralized water home? I really hate to suggest spending cash on distilled or other demineralized water at the grocery store - where it is prohibitively expensive. I'm not talking spring water which is so tasty in good part because of the minerals in it, but that tasteless stuff we would use in a steam iron.

If you can get ahold of a gallon of that stuff, you might try changing a gallon out of the (ten?) gallon tank where Travolta resides and very slowly adding the demineralized water.

If you wish to try this, I'd set the demineralized water (gallon jug?) on top of the tank. I would take some airline tubing, loosely knot it and run it from the jug to the tank, maybe just in the out flow of the filter.

I like that green silicon tubing for this because it is so flexible. A guy wisely tempered my enthusiasm for it by pointing out that it can pinch off. So the green stuff gets used for drip siphons where I want a really slow flow. You should be able to count the drops punking in. The regular airline tubing, which can also be used for drip siphons in a pinch, gets used as airlines.

Your husband - and you - may find this nuts. It may be too little and too expensive to do much on a tank.

It is however, a way of reproducing what the rain does in terms of gradually diluting water. Since fish generally can be moved more comfortably into water with a little greater hardness, TDS and pH, moving "downhill" needs to be gradual. However nature does it every shower and cloudburst. And many fish rejoice by spawning and (as fishermen know) foraging even when the cloudburst is on it's way.

I've mentioned before that as a chemist, I'm a pretty good history teacher. Others visiting this forum, please chime in.

GG please seek additional corroboration on line and in print. Tetra produced an Aquariology series (both in four and in one volume) several years ago which should be available from the library and interlibrary loan. It is dry reading, but has useful info.

Probably several of the aquarium books also address this issue. It's the stuff we all skip over. ;)