Also, I admire your resourcefulness in researching the medications which you are using. Additionally your 2-times a week emergency gravel vacuuming and water changing is to be applauded. You have got to feel a bit like "no good deed goes unpunished."

I was dismayed to learn recently that if I wasn't taking out and replacing close to 50% of my water weekly (or maybe 30% twice a week) in time I would be accumulating a potentially very dangerous and even fatal accumulation of "stuff" in the water.

Heavy plant growth or a sophisticated "denitrifying filter" might mitigate or reverse that concentration of various materials in the water (as will making part of the changing water demineralized water, but otherwise a restart of the tank (keep the gravel wet, introduce a cycled filter and at least 60% water from another healthier tank and maybe use a cycling bacteria pack.)

There seem to be a lot of symptoms you are spotting. The big issue is which to deal with first.

Now this advice is free, It may be worth every cent of it!

First, since it will do no harm (other than cost you some time, energy and water) do even more water changes. I've flogged that story of doing 40% water changes several days in a row on a rainbowfish tank probably much too often on Guppylog. They went from languishing and dying to leaving lots of fertile eggs in a week. The idea is certainly not original with me. Headwater and forest pond fish may experience 90%+ water changes a day. The fresher the water (so long as it was treated with the appropriate water conditioner, let sit a couple of days in open containers to "season" and of about the same temperature as the tank) then the more effective will be their immune systems.

We can help the fish heal themselves of diseases, which we can't even diagnose, with those partial water changes. And too many times, by the time we can ID the malady, it is too late! :(

After doing the partial water changes (and leaving the siphon hose and bucket outside to cook in the sun) I would take a flashlight and look closely at the clamped tails. Velvet is a disease I much more have associated with killifish (from whom the guppies have descended) rainbowfish, Loricariids, Bettas, gouramis and other bubblenest builders. But GL people have taught me that it can clobber guppies too.

If you can see "dust" or a reflective sheen (gray, silver, gold-ish) on your adults, that is probably the problem. I've "bred" a lot of livebearers and killifish (actually they did the breeding) in well planted aquariums where a certain number of fry put in an appearance, are tolerated by the adults and usually grow very fast.

When the fry all disappear at once, I have learned to look for an outbreak of velvet. Otherwise we hardly ever know to look for it or can even see it.

If you feed live, frozen (defrosted in Luke-warm water and then rinsed through a fine-meshed net) or newly hatched baby brine shrimp (bbs) it is easy to overfeed. Even an excess of flake food with shrimp in it can be a problem.

Since the organisms which cause velvet are often in aquaria in a "non-lethal concentration" they can bloom when there is rotting bs (brine shrimp in this case) in the water. Ironically sloppy looking, pond-snail infested tanks seldom have velvet because the snails very effectively scour the premises for any shrimp dinners.

For dealing with velvet, please go to Immediate Help (upper right hand corner) and click to the section about it. Also Googling the net or GL will give you more discussion. Turn off the lights if velvet is suspected.

The most common reason for fish flashing is nitrates in the water. Your test readings suggest that that is not the problem. But there might be something there. The frequent partial water changes might help. Done right, they can't hurt.

By the way, a question I have to ask myself when a malady re-occurs is "did I treat for the recommended period?" Both people treating fish, other pets and taking antibiotics for ourselves too often tend to stop as soon as the animals or we feel better or look better. Often the pathogen's numbers were merely reduced but not effectively eradicated. There is a strong possibility that we or the fish, situation depending, will get sick again. And in the mean time we are inadvertently "trying" to breed antibiotic resistant bacteria or other disease organisms by insufficiently dosing the treatment or dosing for the treatment period.

"... not eating, seemed listless (behavior), gills open and clamped fins with stringy poo... may be examples of what is called Hexamita. Actually with aquarium (tropical fish) the genus of the offending flagellates may more likely be Spironucleus. What ever the genus or species may be, they act much the same and could be treated much the same. There are evidently a community of flagellated little rascals which live around and even in the guts of our fish and indeed most North American fishes as well. During good times, their numbers are held in check by the fishes' immune systems.

When the water gets dirty (it could be fish wastes, it could even be something or some things entirely different in the water supply or in our homes' air) those immune systems are less efficient or are "compromised." The flagellates grow dramatically in numbers. They eat enough of the food in the fishes' gut that the fish are starving. Feces look white and stringy because there is very little left even to excrete! The fishes' throat linings gets so irritated that in time they can't even swallow food! Obviously then it is just a matter of time before they starve.

There are fish foods with Metronidazole in them. However by then the fish too often aren't able swallow food! So look for something with Metronidazole in it which could dissolve in the water. Some of that (like almost everything else dissolved in the universal solvent (water) will find its way through the gills and into the fishes' blood streams.

That may take some time. And while Metronidazole is not among the antibiotics which will most quickly kill off the beneficial bacteria in an aquarium, that situation should be watched. Otherwise we can help effect a cure of the disease and also poison our fishes. Sigh!

*********
By the bye, Log submissions like yours take longer to come to people's attention. The bottom line there is that if you submit a diary it is up and center immediately and not hidden in a log voting queue. (NONE of us read how to post to Guppylog in the FAQs or Immediate Help section 1 until after a couple of weeks.)

Technically a Log (according to the site's software developers) is a multi-page dissertation on some aspect of the hobby or a really complex problem. I thought your situation complex and certainly was deserving of a response. A yes vote seemed like a good idea in order to keep the discussion around long enough to have it. :)