Well first off, those rapid temp swings are extremely hard on fish, but they shouldn't cause pin holes in the middle of the fins. Your tank is large enough you can get a good adjustable 25w heater like this: http://www.drsfostersmith.com/produc...m?pcatid=21322
That's my favorite 25w one but there are others.. some better and more reliable than others. Make sure to use your in tank thermometer to determine temp and not rely on the heater setting as they are usually at least a couple of degrees off. What you do is set the heater several degrees BELOW what the thermometer has the tank at. Slowly click it up one degree until it comes on. From there you can raise a degree or two every hour and no more than 5 degrees per day until he reaches 78-79 range.
You should not keep a snail of that size in a 2.5g. They are extremely dirty and will muck up the water very fast. At the very least you will need to do a lot more careful montioring of ammonia and probably a lot more water changes.
Since you're not using a filter you will never seen nitrites or nitrates. It's ammonia that you want to keep an eye on. With a tank that size you need at least 2 water changes a week - one 50% and one 100%... but that was without the snail. Personally would not keep a snail in that size but I would definitely be doing extra ammonia checks through the week until I got an idea of just how often water changes needed to be done. Your fish should never see as much as .25ppm ammonia. A good drops kit like the one by API is ideal.
Since you're not using a filter you will never seen nitrites or nitrates. It's ammonia that you want to keep an eye on. With a tank that size you need at least 2 water changes a week - one 50% and one 100%. Also with a snail they are very dirty and can be getting the water quite mucky quickly. Personally would not keep a snail in that size but I would definitely be doing extra ammonia checks through the week until I got an idea of just how often water changes needed to be done. Your fish should never see as much as .25ppm ammonia. A good drops kit like the one by API is ideal.
This is how to safely do your water changes:
To do a water change, use a little cup like a plastic solo cup - this cup must be only for him and have never been used with soap or other chems. Scoop him up in this cup (keep him in the cup about 1/4 full of water - it doesn't need to be much because he won't be in it for long) and leave him in the cup while you change his water. To do the 50% use a turkey baster - dedicated only to him that has never seen soap or chems - and drag it through the gravel and try to suck as much of the poop out as possible, in addition to 50% of the water. Use a thermometer under the running tap to get it to be the same temp as the water that is normally in his tank (hopefully you get this around 78-79 soon). When the thermometer says the flowing tap is the right temp, fill back up his tank. At this point, add the conditioner (dose for how much water you change - if you change half the water you add 1.25 worth of conditioner, If you do a 100% water change dose for the full 2.5 change). Float his plastic cup with him in it in the new water. Slowly add a couple tablespoons of the new water into his cup every 10 minutes for at least an hour. Finally, dump him in gently but try to get as little of the old cup water back into the tank as possible. When you do the weekly 100% you will do mostly the same thing except empty his tank fully and rinse everything in it very well under warm water but never use soaps or chemicals. Once it's fully cleaned/rinsed you can refill it and repeat the cup/acclimate phase.
At this point, clean warm water is extremely important to his recovery. You can also add stress coat at 1 ml per 1 gallon to help fin regrowth and you can also treat with aquarium salt predissolved at 1 tsp per gallon for up to 10 days. You shouldn't use salt with the snail. You will replace as much salt as water you take out along with your water changes.
ETA: Can you take the test kit back and get the right one?