Alot of Questions....something for everyone!!

1. Not sure.
2. I won't chance it. A heater is sufficiently cheap - even good brands can be had for less than $20 online - that if even suspect a problem I'd chuck it and get another. Electrocution is a drag.
3. Most online merchants (e.g., That Pet Place, Drs. Foster & Smith) carry them.
4. I've never had any trouble adding cories. They're like the labradors of fish -- everyone is their friend.
5. Watts per gallon depends on the difference between ambient (room) temperature and tank temperature, and the degree to which ambient temp changes in a short period of time. In a room that changes temp rapidly (e.g., the furnace is turned down at night in the winter), you'll need more watts, maybe 5 or more per gallon. In a room with stable temps, especially if room temp is close to tank temp, 3 watts per gallon or less will do.

HTH,
Jim

1) I've heard that NTD doesn't usually spread to other fish, but some tetras aer the ones that would be at greatest risk. I had a heck of a time finding much info. on this when I thought my neons had it. Turns out, I think they were losing color b/c my nitrates were REALLY high, and they seem to look much better now.
2) I agree with just buying a new heater if you can, but I think you could test it out my filling a large glass vase, pitcher or something with water and plugging it in. Use some type of thermometer to see if its working.
3) I've bought several things online from www.bigalsonline.com, I heard so much about them that I finally checked out the site and I've been impressed :dance:
4) You can definately add another cory to an existing school, preferable the same type, but some are not even picky about that (mine are very particular, my julis and my green corys totally ignore each other).
5) I was always told 3-5 watts per gallon, which may vary if you keep plants, but a good heater should adjust regardless of the room temperature. I think I have about 250 wpg on my 60 gallon tank, the temp in the house goes down about 5-7 degrees overnight and the temp. is always constant in the tank.
Hope that helps!

1. my betta caught ntd from some sick neons
any fish is capable of catching ntd if the infectious pathogen is in a high enough concentration. ie: sheep will catch a form of mad cow disease called scrapie from eating recycled cow parts in thier grain even though the disease is not infectious to them normally, same w/ humans.

2. stick it in a bucket of cool water, crank to the highest setting, uplug after an hour and check water temp, if its warm it works.

3. petsmart sells whisper brand impellers for $5 check the front shelfs of the fish podium.

4. if your worried about the existing cory, net him and put him in a qt for a few days the re-introduce him to the tank at the same time you put the new cories.

5. answered already