quikv6

Airstone is a good idea. Some salt may help them adjust as well. As Colu mentioned...what are the KH and GH? Guppies generally prefer harder water, and could be facing a small "shock" if your water was vastly different than the water they came from.

What are your water parameters, specifically? Also, if it is indeed fin rot, you'd me much better off using a med that is more focused on gram negative bacteria, such as Maracyn2 or Kanaplex, as mentioned above. Also, don't underestimate the benefit from clean, clean water with fin rot. Small, daily water changes can really help, combined with the salt and medication.

There's a ridge that snap in and grabs, for a tight fit. It is definitely 2 pieces.

Part number 2 sits in part number 1. It is 2 pieces, and it definitely will pop out.

I'd be focused on getting the 1ppm of Nitrite down, personally. That could cause stress, and lead to illness.

Maracyn and Maracyn 2 together may negatively impact beneficial bacteria.

My recommendation would be anywhere from 25-50%, depending on what level the ammonia and/or nitrite is before the change. The higher it is, the more likely I would be to do 50%. If it's just at .25 or .5, I'd do 25%, and use Prime.

It looks like you may have a bit of nitrite from the test strip pic, but regardless, Ich-X is a great med for the ick.

I'd continue with small water changes for as long as you have .5 ammonia and/or nitrite. Anything .5 or under, I would use Prime to "Detox" it, though this is subject to debate. Ultimately, as the good bacteria builds, you will stop seeing measurable ammonia and nitrite. I have had good luck boosting a tank by squeezing out used filter media from another tank directly into the newer filter/tank.

I think the best treatment would be to get the ammonia and nitrite as close to zero as you can. You really want both to be zero. How long has the tank been up for? It appears it may not have completed it's cycling. Daily water changes (remember to dechlorinate), along with an additional airstone/oxygen will help. I have had success with salt as well. But the main thing is to get the ammonia and nitrite to zero. Welcome to the forum!

Could the municipality have added any treatments to the water? (Extra chlorine, heavy metals, etc)

Is the tank cycled. I noticed 0 nitrates, indicating a tank that may not have completed its cycle. (Or it may just be heavily planted.) A bump in the cycle can cause stress and potential infections.

It depends on plants, if that matters to you. I'd start with 1 TBSP per 5 gallons. You can increase slowly all the way up to 1 TBSP per gallon if needed.

What is the PH and KH out of the tap? You have 0 KH, which can result in a PH crash, which may be why your PH is so low. A crash can cause fish stress and lead to issues. Crushed coral may be your friend, and help increase the buffer (KH), preventing a PH crash.

It looks like it may be a prolapse. It may reabsorb into the body. I have had a couple of fish where that happened. I did a few 15 min baths in epsom salt daily, and fed very lightly, if at all. It took about a week to reabsorb fully.

Salt, and water changes every day would be my suggestion. You can try a different med, or just do a course of Maracyn 2 by itself with the salt and water changes. (Mixing 2 types of meds for bacterial infections is is generally not recommended, and can sometimes stress the fish, which won't lead to recovery.)

Questions 1 through 3 above will really help. Regardless....I think adding a bit of aquarium salt is a good idea. You can start at 1 Tbsp per 5 gallons.

Yes, it can....though eating is a good sign, along with an opportunity to add some meds to the food. Colu had posted a good recipe for adding Kanaplex to the food. I think that would be a good suggestion in this case, along with salt and clean water.

Clean, clean, clean water (Small water changes everyday), along with salt car really help stop fin rot, and allow the fins to grow back. (This takes time.) In your case, I'd consider medicating with Kanaplex as well, since the regression is so close to the body. Once it turns into body rot, the chance of the fish pulling through are much lower.

You may need to treat them with Levamisole or Fluebendazole for wasting disease. It is somewhat common in livebearers.

You are absolutely right in regards to fin progression. Sometimes you can only observe progress by the fact that it's not worsening. Personally, I use a fixed amount of baking soda to raise the PH, though I recommend crushed coral to raise the KH (and in turn, PH). It lends itself to stability overall. I think it would be a good idea to see what your GH and KH is and then go from there.

I TBSP per 5 gallons would be a good start, especially for livebearers like Mollies. I have never used PH-up. Large fluctuations in PH/PH crash can also cause stress and issues. I have no experience with a water softener, but perhaps the hard tap water would be really ideal for your livebearers.

It sounds like you have an immature biofilter, and a tank that may not have completed it's cycle. Meds such as Maracyn can further impact an immature biofilter. Being these issues were most likely due to the nitrite spike and associated stress, I think maintaining clean, fresh water is the best solution, along with some aquarium salt. Small, daily water changes may be your friend for awhile. Also, I don't believe you mentioned GH/KH....but Mollies generally thrive in harder water with minerals.

What method do you use to keep all of those axolotl tanks cool, may I ask?

Hey Charlie...welcome back to the hobby. The idea of a 2 heater tank is a good one, especially to act as a failsafe if one fails in the off position. (Heaters can fail in the on position two, which is where an Inkbird contoller can be a lifesaver.) As for 2 - 100W heaters in a 20 long, I think that may be significantly more than you need. I have an ACO heater in a 125 gallon, 6 foot tank. The basement is in the mid-upper 60's. The single 100W does a great job on its own maintaining 77-78 degrees. Given your 20 long is 1/6th the volume, I think a single 100W will be more than enough (or 2-50 watts of a different brand).