Man, there was some bad advice in this thread...and some good too.
First of all, like several people said, any detectable ammonia is bad. The other fish are tolerating it because they've become used to it, but it is NOT good for them...and they could become sick too at any time. Before you worry about upgrading seaclones, get a handle on what you already have.
Re the heater, if you can wait for about a 7-10 day delivery, BigAlsOnline is very cheap for a good quality AquaVia titanium heater. I've got two (100w around $16 now) and they're dead accurate. No worries about glass breaking (the plastic guard won't protect it..most of them break during water changes when the heater gets exposed to air, continues to heat, and then the cooler water is added). A good QT tank is a 10g from WalMart or LFS @ $10. Add another $15 for heater, $14 for cheap HOT filter. No light is required. If you can swing it, an air pump, tubing and airstone is another good thing to have. Sick fish tend to need extra oxygen during treatment.
I do NOT suggest dumping chemicals in your tank in the hope of correcting what should be done immediately, like water changes.
The next time you need to do a water change and want to heat the water without a heater, just put the container of salt water in the bath tub and fill the tub surrounding the container with hot water. It warms the water pretty quickly. I used to throw a heater in my container, but now that I have an 80g which requires 10-15g of water change, that's how I heat a large amount quickly.
Lowering your main tank temp will not have a positive effect on parasites. In fact if you have ich, most recommend raising the temp a bit because it speeds up the life cycle of the ich WHILE you treat. Lowering the salinity "for a few days" will also do nothing. The treatment to kill the infection requires hyposalinity (think 1.009 or it won't do any good at all) for aprox. 2 weeks. This requires an extremely accurate hydrometer or
refractometer. If it raises above 1.009 you lose the benefit of hypo, and if it lowers, you'll lose the fish. I do NOT suggest you do this in your main tank, as your inverts will die. Fish can tolerate hyposalinity, inverts cannot. You could also treat with cupramine in a QT ONLY, but do not use the tank for anything other than QT/hospital, since the filter, tank, etc. will be contaminated by the copper. A QT tank should be setup barebottom, no substrate or LR. Throw in a couple of small pieces of white PVC tubing for the quarantined fish to find security in.
Based on your description, I have no idea what your clown had? If it was Brook, the only known treatment to have a fairly good success rate is formalin. First a "dip" and then QT tank treated with formalin.
Another issue with clowns this season has been internal parasites. There have been numerous posts on other forums from many owners (and LFS) losing clowns left and right from this. Symptoms are loss of appetite, to completely not eating. Breathing hard, listless, staying at the bottom, and the 1. symptom...stringy white or pale feces. The treatment for this is Pipzine or Hex-Mit in a QT tank.
Good luck with your tank, and there's no substitute for doing your own research...since well meaning people can give some inaccurate advice.