Well... I have been dealing with 2 mysterious guppy problems for some time now... I feel they are connected or the same thing - but they may not be.
The major one is something my local fish stores don't seem to know what it is except for calling it a 'necrosis'... A pale band of what seems to be dead flesh just overtakes a portion of the guppy trunk. They act fine while it progresses then they just die quickly once it hits a certain point. Once I see it I remove the fish from the tank and put them in a 2 gallon quarantine tank.
The second problem is some of the guppies develop a curved spine... Almost as if their back half becomes paralyzed. Their backside just droops down... These fish eventually become lethargic and cannot keep themselves stable in even a weak current thay die shortly after.
The first time I saw the necrosis was from a feeder guppy I had purchased awhile back (bad idea). I wanted to buy some basic guppy feeder mutts that had interesting qualities to them to try and breed them in. All of them died within 3 days of purchasing them (only one from necrosis the others just died with no signs of illness). These fish were never in my main tank... Only the 2 gallon tank. The tank has been bleached twice since.
My tank is testing healthy... Most of the guppies seem healthy and fine - but when it hits one it hits them hard and quick. I do my water changes and I have salt (marine) in the tank (sg
1.00). It is a 55 gallon with some salt tolerant plants. There are alot of snails in the tank I am trying to get rid of with assasins but it is slowly coming into check.
One thing I will note though is some of the snails shalls have white swirls in them and some small snails are pure white. This 55 gallon I received from a friend who had it stored in his garage for a few months over the summer. I rinsed it out and wiped it down... His fish are healthy though.
Let me know if you need any more info.