A bath is potentially worse. It's a small body of water, so a 1/2g error is less dilute in the doseage.

As a bath, no, not a risk to the bio-filter, but more risky for the stock. PP is a very dangerous chemical, and one that should be used as a last resort IMO, especially with Discus and with someone that's never used PP before. I wiped a pond with the stuff by accident on one occasion, a 1g error on caluclating the dose was what caursed it...
Next time do a bath for your koi instead of putting in a pond, get a weight scale and actually measure it. I bet they wern't dead till probably the next day! Was it the blue crystals??? If you do a bath they are not sitting in it for there life, in a pond you need to do a waterchange after. Sounds like a inexperienced person gave you advice on using it. Did it kill off your plants in the pond?? You do a bath, you control how long they are in there for, probably plenty of sites to teach you about it. At least I had the guts to say I have never used it before but still I do understand how it works. Salt is working for him thats good enough then! (Oh next time you get it in your pond treat the fish to baths only with PP and see how it works for you I bet you will have different resaults compared to what you had with the pond issue, its a nutralizer for it) Its probably alot warmer where you are from then where I am from.
Right, in a pond, you by-pass the bio-filter, dose and watch the fish like a hawk for an hour. Any sign of trouble in the hour, or after the hour, you add a quintrouple dose of dechlorinator, and allow it to stew for a further hour before starting up the bio-filter again. I did not use salt along side, and the crystals were white in colour, bought from a chemist. The treatment did not kill the marginal plants, but did have an effect on the ellodia. Waterchanges aren't nessisary after PP treatment, just so long as you dose dechlor to de-activate.
I used a much weaker dose, something like 5mg/l, as it is a bath, not a dip. This is less damaging to the fish, as they are subjected to a lower dose, but for more time. In a dip, you use a high dose and leave the fish untill it's just about on it's last legs before transfering back to the tank, only for the fish to potentially become re-infected, as the tank still has the parasite infection present. The strong dose burns the fishes gills and shortens their lifespan. The weaker bath dose kills the parasites in the tank's water as well as on the fish.
Fungus is easy to clear, using just weak anti-biotics or MG/Formaldehyde. PP is a very risky treatment, even when done correctly, and totally un-nessisary here.
In high doses, yes, Iodine is poisonous to fish. It is however a component of sea salt, so you can't avoid adding it with pure sea salt either, in fact, you are more likely to add it with sea salt. Table salt here in the UK is usualy Sodium Chloride and some anti-caking agent only. There are a few brands that a pure Sodium Chloride, and this is what I use for baths myself when required. We are dosing small ammounts however, so Iodine in the salt isn't too much of an issue, it is not toxic untill above the levels found in the sea to fish, and we are adding to about 1/9 of the strength of sea salt. The Iodine won't be concentrated enough to caurse issues if presant

Anti-caking agents, well, they are a different matter.
You might as well to a Ammo bath then a Iodine bath for them, but I never knew what your salt was made of till now. Our salt here has pure Iodine and you can buy salt gaurenteed not to have iodine in it. Real sea salt has no Iodine! If you buy sea salt with iodine its not pure or natural then look for products that say natural on it. All our sea salt here has no iodine in it period! From over there I don't know I don't shop at your lfs or grocer so I can't say but you should stay away from Iodine period. Just because its sold in a lfs doesn't always mean its pure sea salt or even real for that matter.
As a reefer, I can tell you that Sea Water naturally contains 0.06ppm of Iodine at an SG of 1.026. That's more Iodine naturally preesent than Phosphate... Many reefers recomend dosing Iodine to improve coral health and colour, though dosing it is complex and risky. If you Sea salt dose not contain Iodine, it isn't sea salt

(Have a look at articles on-line by Randy Holmes-Farely if you don't believe me. Holmes-Farely is one of the leading reef aquarium chemistory authors ATM. There are also several scientific papers on natural sea water analysises that will confirm his claims.)
Technically, anything living on a host is a parasite
Not if its identified on what it is!, most can be assumed its a parastie the definitions are differnt of them both simular but not the same. This type is a parasite fungi
A parasite by definition is any creature living on a host. It does not matter if the species is identifyed or not, of it's living on a host, it's a parasite
All the best
Rabbut