Water quality tests fine but my fish keep dying!

Question
Ive had my 70l tank set up for over a year, and generally it seems fine.
I then get spells of two of three fish (tetras, guppies etc) dying from what seems to be dropsy- they suddenly swell up, go pine-coned and get popeye then die.
I change my water (about 12L) every other week with a good gravel clean (i have a plec too)and monitor the water and apart from a slightly high nitrate which is typical of the water in general where i live its fine- ph 7.2, no ammonia or nitrate.
I use an air stone constantly to keep the tank well oxygenated, i have plenty of rocks and plants, both natural and real for the fish to hide in and maintain a temp of 26C.

Im not sure where i am going wrong! Ive recently bought internal bacteria specific treatment on the advice of staff where i got my tank from, but they couldnt think of any reason as to why this might be happening.
Also, ive notice a few of my guppies seem to get blisters in their tails that come and go and i wonder if this might be linked?

Answer
Dropsy is an odd disease in that it is seemingly not contagious.  It usually strikes one fish not many so I wonder if in your case if the dropsy is the symptom rather then the disease.  I liken Dropsy to congestive heart disease in humans with the exception that in fish the entire body is affected and in humans it is primarily the pleural cavity that is affected.  If the two diseases are related, even just slightly, what would cause the disease to progress in fish?  In humans the congestive heart failure CHF is sometimes proceeded by warnings signs such as wheezing, swelling in the ankles, etc but in fish those signs would not be noticed.  CHF in humans is caused when fluid is not removed but what would cause that to happen in fish.  Two things come to mind... a constriction of vessels either by parasites such as worms, or massive organ failure.  Parasite treatment has been marginal at best and in most cases the fish dies.  If organ failure such as kidney failure what would cause that in fish... genetics?  Diet? or something environmental.  You mention that some of the fish have blisters and so I am wondering if it is possible that the fish may have encountered some toxin like household cleaner, etc that may have triggered these events. Poison or toxins would cause organ degeneration and the end result would be dropsy or something akin to it.  Your water chemistry is perfect except for the nitrate but two things come to mind here.  One the nitrogen cycle begins with ammonia which becomes nitrite and then nitrate....so the fish may have been exposed to something like a cleaner that has ammonia in it of a consistency that would cause blisters and potentially organ failure.  The second questions that emerges is that aquarium test kits only test for a few chemicals... ammonia, nitrate, nitrate and then pH... so what else could be lurking in the water?  I am leaning more and more towards some kind of toxin because the disease is reoccurring, you have more then one victim, and the victims are seemingly smaller fish.  A dietary issue would effect just one fish, whereas genetic issues would effect one kind of fish, but a toxin would cross those boundaries and effect just those fish who encounter it regardless of genetics or diet.  The other issue to consider and this is more of the clincher for me... is that the disease almost has a trackable cycle... which dropsy does not.  Questions to ask are:  How often do the fish develop dropsy?  Is there a correlating event such as pest control or ant/fly spraying that occurs on this cycle, What do you clean your home with? do you have a product that you use on the same schedule that the fish are getting sick?  I would wager that the event happens prior to the dropsy showing up. So that the fish go through a small stage of disease process where they have signs and symptoms that are not noticeable so when you ask your questions think about a week before you notice the dropsy.   I hope this helps a bit.  I know its kind of random but that is just how my brain works.  I would feed the fish live brine shrimp every other day to help with their diets... the bring have tiny shells that act as fiber so it helps to clean them out and has an effect on internal parasites.... please keep me posted... dave