Things that look like tail nibbles are often the result of a bacterial infection. Resting at the bottom is another sign that would suggest this. But before you go and blaze with antibiotics, keep in mind that it'll slow down (or destroy) what cycle bacteria you've managed to establish. (The packets say that the antibiotics wont interfere with the nitrifying (cycle) bacteria, but that is assuming you have a strongly established cycle that can withstand a hit from the antibiotics. You dont want to mess with this when the bacteria have a fragile foothold.)
I do think you should reduce the size of your water changes, so long as the ammonia/nitrate dont get up too much. 75% change is more what I'd consider to be at "emergency" change level. You dont want them to be more than 0.5 or 1.0 tops. Adjust your water changes so that neither level goes much above about 0.5. Try reducing to changing just 5 or 10 gallons at a time. Hint: home depot orange 5 gallon buckets are great for measuring and preparing water. Two buckets == roughly 30% of the tank and the water conditioner instructions often give capfuls per 5 or 10 gallons. Don't be afraid to test more often when changing the amounts.
Doing large water changes itself can cause stress, so its a case of weighing up the consequences either way. You're damned if you do, and damned if you don't. High ammonia/nitrite levels cause stress and injuries, and large water changes cause stress. And besides, water changes prolong the establishment of the cycle to some degree because you're unintentionally removing part of the helpful bacteria. The stressful part of water changes is that it causes changes in the water chemistry, eg: pH (acidity) level. Fish react to pH changes relatively poorly. Most modern water conditioners seem to change the pH level, so doing a massive water change cause a sudden pH change, and a temperature change, and probably scares the fish silly. Smaller water changes are routine since they dont get much chance to suddenly change the water chemistry.
Another unfortunate fact is that guppies from pet stores have a nasty habit of just dropping dead a week or so after you get them home. The shipping/transport/new-tank/food changes/etc really knocks the socks off them. That is, if they don't die in the pet store. For example, I've been asking around locally and it seems that the local pet stores are having 25%-50% death rates within a few days of getting a shipment from one of the the main san francisco bay-area distributors. In the pet store tanks over the last two weeks, I've seen columnaris, sitting on the bottom, ich, tail rot, internal bleeding. Of the fish I still have in quarantine, one has just developed (real) dropsy. Another has columnaris. Three went from "just fine" to dead in 8-12 hours. ALL the blue-tail grey female guppies died within 24 hours of getting home - even though they came from three different pet stores. Anyway, I'm seriously glad these poor fish are in quarantine still.
One thing that I was told over and over again was to put some salt in the aquarium. It took me a while to give in and try it, but it seems to have helped settle things down. I'm still using 1 tablespoon per 10 gallons of water. Since I change 10 gallons at a time, I add another tablespoon with each change. Make sure the salt is dissolved completely first. I take a small cupfull of water and dissolve it in there, then pour it in the tank. Don't let your fish contact solid salt crystals - that would be bad. However.. you have other fish in there, right? Plants? While guppies thrive with a bit of salt, some other species don't appreciate it all that much. Plants don't like lots either. In case you were wondering, the salt slightly irritates their skin which stimulates the body slime production. That is a good thing because it is a front-line defensive measure against illness. FWIW, guppies will tolerate massive amounts of salt in the water. Much much much more than 1 tablespoon per 10 gallons. So don't worry about the possibility of overdosing. Use actual aquarium salt or "rock salt", not table salt, nor iodized salt. Iodine is BAD for fish. Many table salts have an anti-caking agent in it that is actually cyanide!!! Seriously! Yellow prussate of soda is "sodium ferrocyanide" and it'll kill your fish in short order.
Salt can be used as a treatment as well as a low level preventative. I believe the theory is that small cell organisms do not have the ability to adjust to different salt levels quickly enough and it messes up their osmo-regulation and they die. Do some research if you want to try this. I believe the usual instructions are: add 1 teaspoon per gallon, twice a day. ie: if you have a 5 or 10 gallon tank, then you'd be adding 5 or 10 teaspoons in the morning and night, for three days. And then a few days later, start reducing it via water changes. Don't change the salt level too suddenly or you'll mess up the osmo-regulation of the fish and kill it.
If you decide to try antibiotics, there are a whole host of choices. But, a personal suggestion.. Keep a spare 10 gallon tank handy. Empty if you have to. They are relatively cheap. You don't want to use triple the amounts of medication for your huge tank if you don't have to, and you don't want to risk injuring your biological filter in the main tank. If you keep it empty somewhere out of the way, you'll have to use some ammonia absorbing chips or some other device to absorb the ammonia. One fish won't produce all that much though. You can siphon 10 gallons straight from your main tank to reduce the stress of moving a sick fish to a hospital tank. And in the process, the main tank gets a bonus water change.
My second personal suggestion regarding antibiotics is to use something that you can get more of readily. This means something you can get easily at your local pet store. Or if they're sold out, another one will likely have it. This pretty much restricts you to mardel's maricide etc products, and AP's version. Maricide + Maricide-two cover both gram positive and gram negative bacteria. Some others do both positive and negative in the same dose. Others include TriSulfa and Tetracycline. AP makes versions called triple-sulfa and so on. (Triple-Sulfa is a slightly lower dose compared to TriSulfa). I keep a small stock of maracyn and maracyn-two (and a couple of others) handy so that when I notice something at midnight I can start treatment right after researching, instead of letting the bacteria eat my poor fish for another 12 hours while I'm waiting for the store to open. And since my stocks are small, I have to get more soon. Hence the comment about using widely available medicines.
Anyway, I'd personally put the death down to something that came home from the pet store, with the possibility that stress helped. Don't beat yourself up about it, you worked so hard! Much more than many others would have done.
Think of it this way.. The chances are that all five of those fish could have been ammonia poisoned and dead by now if they'd gone to somebody else. Especially given the poor quality (mis)information that new hobbyists are given at pet stores. I've lost count of the number of people I've spoken with over the last few months who said "oh, we tried a fish tank but they all died in a week". Aaargh!!!!
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