1) In a 10 gallon tank, I have 3 male platys and 1 male molly. In a 26 gallon tank, I have 10 neon tetras and one guppy. In a 20 gallon tank I have probably about 20 female platys, which I know is too many but they gave birth a lot and I'm trying to find someone to give the babies to so that the nitrates can stay in normal ranges.
2) For all the tanks, I use tap water. All have 0 ammonia and nitrites. The ten gal has about 25 nitrates, the 26 gal has less than 20, and the 20 gallon has about 80 (I've done a big water change since I measured this though since it was really high)
3) I give them the hikari fancy guppy/livebearer food. I feed them twice a day, as much as they can eat in a couple minutes.
4) I've had them all for over a year which is why I find it weird that they all died at the same time because they weren't new and were in separate tanks.
5) I do 10-20% water changes at least once a week in every tank.
Also, since I wrote the original post, I noticed a guppy in the 26 gallon tank has bubbles in his top fin, so I don't know if it's a fungus or Gas Bubble Disease, which came up when I looked it up. I don't know if it is related because one of the websites I looked at was talking about barometric pressure as a possible cause, so I wonder if there is something up with that in my house and that could be why it affected all the tanks, and they just didn't have visible bubbles like the guppy.
I do use the same equipment for all my tanks, but I do water changes on separate days for all of them and rinse it in between so I find it unlikely that something was passed between tanks.
Before jumping to disease conclusion with the guppy, are you sure the bubbles were IN the fin vs on the fin? If they were IN the fin, make sure you do not have a lot of tiny bubbles in the tank ( either from an airstone or filter return. ) Tiny bubbles are a frequent cause of gas "disease" due to the fish swallowing them when they eat. If you have them, you will need to either change the air stone to a large bubble stone or adjust the filer return in the water so that it does not create tiny bubbles on the return.
While the Hikari Guppy food is good for keeping their color up and the fish healthy, it should be PART of a diet that includes other foods as well and not the only food you feed. A diet solely of dried foods can lead to constipation or internal infection which can be the precursor to Dropsy. Dropsy is more a symptom of a problem more than the cause of the problem. It's from the fish's inability to regulate their liquid exchange from the body. This is due to organ failure. So their diet should include vegetable matter ( either flake or frozen or infused in brine shrimp) as well as foods with Chiton ( which is a roughage) to help keep the fish's intestines moving food along. Chiton is found in the shells of foods like brine shrimp, mysis shrimps, daphnia and water bugs. I would change your feeding to the Guppy/livebearer foods for one feeding and one of the other foods I mentioned for the second feeding. Even better would be say, Guppy food and Spirulina infused frozen brine shrimp on one day and then A general good quality flake food with either Spirulina flake or frozen Daphnia or frozen Bloodworms for the second feeding on the next day then continue to rotate the different foods so that the fish has a diet of 3-5 different items. In my Hatchery, I fed 3 times per day. Morning was general flake. Afternoon was a higher protein flake or live worms and evening was always frozen brine shrimp. ( There was no such thing as algae infused brine shrimp or else I would have used it.
) Every morning, the floors of my tanks had generous amounts of "Poo" that the brine shrimp helped produce.
Proof that their " pipes" were flowing well.
As for the stress factor: While separating the male platies from the females is a great way of controlling population, a colony of males only will sometimes fight for dominance so you really need to have them in a tank with a lot of hiding spots or other fish in there to distract them from fighting each other. 1 extra Molly is not going to cut it for distraction. Use lots of taller frilly plants for good hiding ability.
Unfortunately, females can also have issues with pregnancies and when giving birth which can also kill them so there is no real way of controlling that. The best you can do is keep them in as healthy a condition as possible to help reduce issues.
Truthfully, if the Guppy is a male, pairing him with any Tetra is not the best idea as any tetra specie can be fin nippers and since the male guppies have that enlarged tail that does hinder their swimming abilities somewhat, they are sitting ducks for fish that can swim at normal speeds. Female Guppies at least are better, faster swimmers so they can fight back better. Still, not the best pairing.
As for your equipment, unless it is all bone dry before using it in another tank, there is the possibility of cross contamination. It's better to rinse nets in salted water or a bleach solution or there used to be a solution called Net Safe by Jungle Labs which is a sanitizer or left in the sun to dry completely. Hoses should be rinsed, inside and outside, with either hot water or a mild bleach solution.
I'll add this: I think, based on what you have told us, that it's more coincidental that you have 3 fish in 3 different tanks having similar issues than a disease that was cross contaminated. Considering that 2 tanks are both platies, both could be having dominance or dietary issues. The fish with Dropsy could very easily have been a dietary issue. Usually, when there is a water issue, all the fish suffer, not just one or two. Sure some fish may be able to hold on longer than others under bad conditions but they too will visibly be effected.
That's how I see it.
