Adding less than a teaspoon of granular salt per real gallon, in an aquarium, is not likely to be a problem with guppies. But if we keeping adding salt to our changing water like that, we can do great harm as the salt builds up in that tank. (This sounds like that Winter Mystery Deaths log again.) This has happened, though I would be hard-pressed to document it at the moment.

If Rift Lake aquarists use sea salt rather than the calcium-magnesium based cichlid salts, they can kill those fishes too. Jodybr is probably very familiar with cautionary stories about that.

If straight salt is added to really soft water, it can create an osmotic imbalance which actually seeks to draw crucial minerals out of the fishes' bodies! That can be the origin of another one of those "mystery fish deaths."

Salt is used as a treatment in many fish medicines because it actually irritates the skin of fishes. The irritation causes the skin to generate more skin slime. The skin slime can fence out or encase some ectoparasites, helping the fish to defend themselves.

Salt doesn't change the hardness of the water or its pH at all. It has a huge impact on the increased TDS (total dissolved solids in the water). Dropping a fish into water with several times the TDS (which includes salts like sodium chloride, the hardness salts like calcium, magnesium and some others, fish poop and other nitrogenous goodies, dust which blew in, ad infinitum...) can be a traumatic as dropping them in water a couple of pH numbers different or in water considerably softer or harder in hardness. If the fish doesn't die then, it may later because its respiratory and immune systems are severely messed up.

Ocean water is currently an average of 3.5% (by weight) percent dry matter. Most of that is sodium chloride. However ocean salt is composed of 80 elements - minerals.

Solar Salt is basically the production of table (or feed) salt by drying ocean salt in evaporation ponds (ironically also a habitat of brine shrimp - for a while). Probably it would be the best "salt" to add to aquaria. Not trying to necessarily plug the health food stores. One can also stop by an agricultural supply story and buy a 50 lb. bag of cattle feed salt for a couple of dollars.

Iodized Salt is ok. Not only has it been a boon to the health of people, especially in the more industrialized countries (vis-à-vis iodine deficiencies), but the addition of a little to an aquarium isn't likely to hurt guppies. Hard water fishes can actually suffer from an iodine deficiency. This is a malady sometimes knows as fish goiter.

http://www.curezone.com/foods/saltcure.asp
Is a site dedicated to making a case for using unrefined sea salt rather than the "refined" and definitely altered commercial table salt. You make your own conclusions about that issue. But the site has some interesting data about sea salt and what is in it. One of their arguments for people using unrefined sea salts is that too many needed trace elements have been taken out of it. That is not too different from what we have been saying on GL about using straight Sodium chloride, as opposed to marine aquarium salt or even better, a cichlid salt, if the water needs more "mineral" in it.

http://saltaquarium.about.com/cs/seasaltmixes/l/aa090503b.htm
is an article on the chemical make-up of commercial marine salt mixes. Interesting the differences between them. Also striking is that all of them have levels of ammonia and nitrates higher than average seawater. That may have no influence or a modest effect on aquariums, especially crowded ones.

So yes Fisher, has the amount of salt built up too much in a tank, is a great question. It probably is not the problem in this case, but it should usually be considered