Okay, apparently your fish store didn't fully explain what cycling an aquarium is because what you did would not cycle a tank. The good news/ bad news of that is that what the fish have is not related to the tank not being cycled.
So let's start with the fish part: That redness looks to be a bacterial infection on both fish. When you are dealing with Male Guppies, there can be some fin nipping which can get infected but the timeframe you mentioned ( ie new on last Fri Jan 9 and death on Mon Jan 12) says to me that they were infected when you got them and your water's hardness ( or lack of hardness) just exacerbated the issue but not caused the issue. Next comes the Bactonex. Since it's a new product to me, I checked the ingredients in it and the methylene blue in it will greatly inhibit or kill nitrifying microbes so that should have been used in a separate hospital tank and not the main tank. The good news (?) is that there probably was no nitrifying microbes present ( we'll get into that in a bit ) so "no harm no foul" but it's going to take a number of days for the medicine to work. If you notice the other fish picking at this one's tail, you will need to separate the fish with either a tank divider or another tank.
On to the tank: ( please excuse me if I am giving you information you already know. It's better to give all the info vs just some of it. ) When you are cycling a tank, you are creating a bed of nitrifying microbes. This is also called the Biological filter bed. This Biological filter bed will contain 2 species of microbes. One will convert the ammonia that comes from the fish into nitrites and the second microbe converts the nitrites into nitrates. This can be done with fish in the tank or without fish in the tank but using another ammonia source. What you were trying to do is called a " fishless cycle". In order to do that, you need an ammonia source and the "bacteria in a bottle " product. There have been studies done on these many "bacteria in a bottle" products and sadly Stability was not included on the list of good products. Only these products were found to actually contain live nitrifying microbes: Fritzyme and Fritz Turbo ( made by Fritz Aquatics) and "Tetra" brand's SafeStart and Dr Tim's one and only. ( Dr. Tim's is the same formula as Tetra SafeStart so it's an assumption that it too would be included on this list.) So this is why I say you probably had no biological filter running when you added the Bactonex. So for now, you are going to need to follow the directions on the Bactonex as per water changes and redosing schedules. If you have not had your general hardness tested or don't have a GH test kit, I'd have your local shop test your water hardness ( GH) and if it less than 90 ppm/ 5 dGH, you should add aquarium salt ( or any NON IODIZED salt) at a rate of 1 tablespoon per 11 liters of water. If the GH is over 270 ppm/ 15dGH, you should do a partial water change and use RO or distilled water to drop the hardness to a more suitable level for Guppies and
don't add any aquarium salt. From there it's going to be a wait and see how the fish does with the medication.
As for cycling the tank afterwards: This is a picture of the nitrogen cycle in graph form.
View attachment 392916 As you can see, the beginning of the ammonia and nitrite cycles look the same at the start and at the finish. The only way to know that you have biological activity is to test and see them both go up then down and once the nitrite is going down, you should be seeing an elevated nitrate level. ( There's no real need to test nitrates until you see nitrite coming down. )
With fish in the tank, you are now going to be doing a " fish in" cycle which means you will need to keep tabs on the ammonia and nitrite levels because you don't want either to get higher than .25 ppm or a combined level of .5 ppm. If you are using one of the "bacteria in a bottle" brands I listed, your levels may never get that high ( which is a good thing
) but if you are not using them, you will need to do water changes to keep the levels in the safer zone of under .25 ppm. Do not use any products that claim to "detoxify" ammonia and nitrite if your levels are high because they only do so for a short window of time ( 24-48 hours) so doing a water change is a better, less expensive method of reducing ammonia and nitrite.
I know, this is a lot of information to absorb but it's all important info to know so ask any questions you may have.
Lastly, just an observation because I worked in the pet industry for over 45 years, I would consider finding a different store to get your fish from. Your sales clerk should have seen that your tank is not cycled when he/she didn't see any nitrates. If the Stability did it's job, there should have been some nitrates when they tested before selling you the fish. Between that and the probability that they sold you sick fish, I would be hesitant to trust them again. Just my opinion.
Hope this all helps.
