I look forward to seeing the pictures, that will help alot.
As for the cycling I mentioned... with a 0 nitrate level, the tank is not cycled or it is over cleaned. Either way, nitrate is the end of the cycling process, and when it finishes, if there are animals in the tank, there should be something for nitrate. Also please remember that while ammonia may convert to the less harmful ammonium, it still also converts to nitrite as it breaks down, and nitrite is also very harmful to the fish.
If someone is reading an ammonia level of something like 1.0 - 2.0 but at a pH of 6.0, they may think this is ok and safe enough for the fish. The problem comes when that high ammonia level begins to break down to nitrite... all of a sudden the tank is a toxic mess and it catches them off guard. So keeping up on those small water changes is still important. If you have the ammonia reading in your tap water, if possible to let it sit out for 24 hrs before using it, by all means do so.
Once the tank is cycled, a .25 ammonia level going in during water changes should be accommodated by the bacteria culture and broken down quite fast. As the tank matures it should find balance. Another option for you is to filter your tap water before using it in the tank for water changes.
A rubbermaid tub with a hang on filter that has carbon in it is all that would be required. The carbon should remove that small amount of ammonia within 24 - 48 hrs as long as the carbon is kept fresh. With a hang on filter you can save old nylon stockings, cut off the toes, fill with carbon and tie shut. Lay that in the filter box in place of the cartridges and you're done. This would allow more carbon, faster filtering of ammonia, and only needs to be changed once or twice/month, depending on how much water you are adding during the course of a month. I have used this method in the past due to bad tap water, and it works nicely. Once I use water from it I fill it again immediately so it is ready by the next time I need it. It is also very easy to add a heater to the water so its always the right temp... just be sure to keep a cover on the tub to avoid dust and lint and other nasties in the air from getting in. The cover of a rubbermaid can be cut just as an aquarium cover can be, to allow space for the filter but to keep the remainder of it covered. Then just snap it on and its done. The cover will help to maintain heat in the water, thus the heater is not running constantly and also helps to keep kids and pets out.
Another option that is a bit more costly in the long term would be to use Prime for your water conditioner, mix very well and let sit for 10 - 15 minutes before adding it to the tank. Some people don't like the sulfur smell that it has, and in the long term would cost a bit more than the filter method, but its less mess and less time consuming. Prime will help to eliminate the ammonia in the tap water before you are using it in the tanks. I am not big on chemical fixes, but in your situation it is something that becomes a valid option. Prime is a good quality water conditioner that works well for chlorine, chloramines, heavy metals, and ammonia.
When it comes to the cycle you start with the ammonia, which feeds the bacteria. The waste product of those bacteria is nitrite. More bacteria feed on the nitrite which further breaks it down to nitrate. An ideal fully cycled tank should read 0 ammonia, 0 nitrite, and nitrate under 20. When enough bacteria have populated a tank to handle the current waste load, these levels stay pretty stable as the ammonia is broken down so quickly, as is nitrite... all that we find when we test is nitrate. So removing the extra ammonia, even at the lower pH is still important.
When starting a new tank and the only thing showing a reading is ammonia, it shows lack of bacteria population to break down that ammonia quickly, and nothing converting it to nitrite or nitrate yet. This indicates the tank is not cycled, either from still being very new or from over cleaning which can destroy the needed bacteria population and takes time to replenish.
It might be a wise idea to step those water changes back to every other day to give the tanks a chance to fully cycle. Be sure not to gravel vac or change filter medias during this time. I would also keep a close eye on the water chemistry during this time, testing every few days until the cycle is complete or if any of the fish appears to be in distress.
Your biotope tank sounds like fun, what size are you planning for it? I can't wait to see photos of it! I am currently at my limit on space right now with 14 tanks. The last one is still in the beginning stages, going to be a 29 gallon biocube done freshwater with sump. I am making some modifications with lighting and screening over intake slots because I am hoping to find the scarlet badis I've been searching for forever. I never thought it would be so difficult to find them.
The pleco should love a biotope habitat, so be prepared for him to grow very quickly. If he's a standard pleco he's gonna be a very big fish with an equally big appetite. You may find that for such a habitat you will need to add new wide leaf plants regularly. But oh, he's going to have so much fun in there, lol.
I will do all I can to help you get him well again, and will await your photos of him before we go any further. One last suggestion for you... invest in a heater guard or use rock work to block off the heater so he can't lay against it. It would be plenty safe and also beneficial for him in his current condition if you could notch the temp up to 80 - 82, also. The warmer water temp will help boost his immune system to fight any infection that may try to set in, and it may also draw away from his desire to spend so much time close to the heater because he won't be seeking the heat.
Have a good night!