Your pleco ((maybe Hypostomus plecostomus, but the genus if you check wwwplanetcatfish.com is enormous, numbering possibly more than 100 species)) is typical of most of the suckermouth catfish (or Loricarids) in that they don't encounter any appreciable sodium chloride and not a whole lot of other minerals in nature and, while they will tolerate maybe a level tablespoon per 10 gallons of water, one could be advised not to get too generous in salting their aquarium. Probably they could tolerate twice that much, but really the salt, unless used as a "tonic" for messed up skin, is to buy us time (by delaying nitrogen poisoning) to pick up the frequency of partial water changes.

The ichthyological opinion is that are now 35 families of catfish. I'm guessing (and may need to be corrected) that only one catfish family is marine, the Ariidae. And the industry sometimes continues to sell those little "forktails" or shark catfish to fresh water aquarists. (Species depending, they can grow to 8" and up to 6 feet long.) But the other zillion catfish in the hobby are not used to any salt of the sodium chloride persuasion (as you know, NOT table salt) and don't need to be "pickled." ;)

http://www.planetcatfish.com/shanesworld/shanesworld.php?article_id=174

You are wise to keep doing those partial water changes with treated, "seasoned" water, as many times a week as you can. Since guppies tend to come from headwaters (small streams at the beginning to River systems) among other places, rain run-off and seepage from the water table, at least in the wet seasons, may be more than 90% a day. We will probably never reproduce what is going on in the wild, but we need to give them the best possible water quality. And fancy guppies are more sensitive to dirty water than common guppies.

By the way, both in South American and in many places in the US, the hot, less wet/dry months don't see as much new clean water entering the water systems and what do you know, the fish carry a lot of diseases. Recently read an article documenting game fish in the US that often develope columnaris during the hot, oxygen poor months of July and August. When mother nature doesn't keep up the water changes, thousands of fish may sicken and die.

Venezuela's wet season is roughly May to November. One of the Amazon Basin forests experiences a wet season from January to March. The Balbina area, 180 km north of Manaus (cardinal tetra country) is February to July. The reason I mention these is that though a lot of the popular aquarium fishes are now farmed, the rarer stuff, such as many suckermouth catfish (with the exception of the common plecos, some bristlenoses and the unbelievably expensive zebra plecos) are from the wild. By the time they have come through the commercial chain, inadequately fed, they are sometimes swimming museums of diseases. Most collecting is after the rains are stopping, waters are receding from the flood plains and that year's YOY (young of the year) are maturing. If they were collected in receding waters late in the dry season (look for the month or two before the rain) they may already have been in danger of some malady and imports then may need to be avoided or seriously treated with an anti-parasite treatment.

As you probably know, even with your pleco (perhaps from a fish farm pond in Florida) it will often come bearing parasites and need quarantine and a preventative treatment. It is a wonder that anything survives those fish if they are directly introduced into a community tank! Yours has caught on, it may not grow more than a foot long, but could theoretically reach to almost double that length and live 15 years. ;)

And then there is food. I was feeding our growing golden bristlenoses little slices of sliced, lightly microwaved zucchini. Whoo-wee! The aquarium clouded right up! Either the uneaten zucchini or the stuff whizzing through their systems, really set off a bacterial bloom. They are having to settle for veggie wafers. And even so, that tank has to have partial water changes every few days. They will soon be moved to an aquarium twice the size.

The somewhat vegetarian Gambusia vittata that share that particular aquarium, seem to be doing fine, but I fret over their odds of sickening if that tank was only given a 50% change once a week. As you know, who is in the aquarium, how big they bulk, the total population, the size of the aquarium, the effectiveness of the aquarium as a biofilter, what the fish eat and how recently they were acquired all come into play.