You mention that there was a pH plunge. Did you deduct that from suddenly flighty behavior (acidosis or "crazy man's disease") or from a pH test that you did?

The rotting food is the biggest problem and cause of poisoning. Siphon it out. If you have a gravel vacuum, use that to get some of the gravel cleaned of some of the gunk down there.

(In time make sure that you have a gravel vacuum. They really are essential if one uses gravel.)

In fact, a couple of drops of water conditioner will temporarily bond with the extra ammonia there. (It really doesn't get it out of the tank, as some advertisers suggest). But it gives you time to make partial or complete water changes.

I applaud you for having some water seasoned for a month and treated. Gradually siphon that into your tank. I use cut off gallon water jugs, a piece of hard airline tubing leading out of the
jug and some green silicon airline leading over the jugs' bottom edge so the water slowly streams into the tank. That should gradually and less stressfully add better water to the tank.

The algae isn't a problem if the light is on, only if it is off and the algae is rotting. In fact, with the light it will used carbon dioxide and ammonia (from the fish waste and decaying food) actually somewhat improving conditions. Algae is only really dangerous when decaying. I do not like it because it can threaten plants I want in there and can be unsightly. If healthy, it is not dangerous to the fish. Rare exceptions are a red algae in salt water and that green-blue slime (Cyanobacteria) in freshwater. And even then, if partial water changes are kept up and there is adequate light, they are not initially a big threat. (And with improved tank care, more partial water changes and competition from higher plants, they may go away.)

In your case I would have then put a tiny bit of water conditioner into your holding container(s) & refilled them with tap water. If I could leave those containers where the water was almost as warm as the water in the tank, I'd do some more gravel vacuuming tomorrow, empty half the tank and fill it with the new water, refill the containers and condition/ season some more water.

If that pH business is a problem put in a level 1/4 teaspoon of baking soda for a quick, short-tome fix. That should buffer the system a little. If it is a 10-gallon tank and the pH is still low (below 6 or a sudden drop can be troublesome for guppies.), perhaps add a second 1/4 teaspoon.

In a previous life I was a school teacher and noticed a tank like yours. The residents were scooting around in erratic and tipsy patterns.

1/2 a teaspoon of baking soda was sloshed in to buffer the pH up and the tank was tightly covered so there would be no panicky jumpers.

The box filter was running actively. That both adds oxygen to the water and forces some carbon dioxide (carbonic acid in the water) out.

I left for school. After the school day I pulled the four pairs of breeders out of that tank, took all of the water out, lightly rinsed the gravel and filter, keeping them wet. The tank was filled 50% with water from another healthy tank and 40% treated water.

Much of the water holding the fish was carefully poured off into a bucket. Added as much tank water as there was water still in the holding container. After 20 minutes, all of that water was drained into the bucket. Water was added again from the tank and the holding container was covered again. After an interval of 20-30 minutes the fish were returned to the tank.

If the fish in the jar were showing any distress, I would not have gradually acclimated them to the new water. In that case their water would have been carefully poured through a net (so as to wet the net) and the strain of Fp. gardneri quickly transferred to their rehabbed home

Your problem with putting the fish in tap water and later adding Tetra Aquasafe is that you did that backwards. You also added some of that nasty (perhaps ammonia rich) water from your aquarium.

Best might have been to just add the to the "seasoned water."

They don't even need to be fed for a day or two.

If floating plants (cheap hornwort) can be added and kept alive, that will soak up quite a bit of liquid waste material.

By adding the chlorine treated water first, the chlorine could burn away some or even a critical part or all of their gills. They now can't breath as well as they should or are even suffocating. :(

The Sera Aguatan looks like useful stuff in several ways (though the write up said nothing about temporarily neutralizing ammonia. It does have several things to recommend it other than the ammonia issue.

You did well to treat water ahead of time. There is no law that says that a tank must be full - though obviously a power filter hanging on the tank side can't be used. If your box filter is empty, use it anyway with a stone or four marbles in it to hold it down. It will do most of what an airstone will do, maybe more.

To my surprise, virtually no oxygen comes from those little bubbles rising from airstones. What the little bubbles do, as opposed to fewer big bubbles, is cause more water to rise. When it gets to the turbulent surface it will release some carbon dioxide and absorb some oxygen. So too a HOB (hang on the back) filter, even if it is empty, does a great job of circulating water to the surface.

Next time, if you have filter floss or a filter pad, only rinse it until it is a lighter gray
and all the pieces of dirt are out if it and keep it wet. (Purist would have you put some aquarium water in a bowl or tray and rinse it there so that the chlorine can't get at the beneficial bacteria.) That leaves some beneficial bacteria to colonize it and get an efficient nitrogen cycle going again.

Cleaning excess "stuff" on everything off is good, but I would disagree with scrubbing absolutely everything off. If rocks and gravel are not that really foul black (throw them out if they are) then knock extra algae and dirt off of them. If they are not allowed to dry out or get put into chlorinated water, they will keep some beneficial bacteria with them.

By the way, unless really caked minerals are on the side of the aquarium, in a situation like yours, skip the vinegar unless you really rinse everything. Vinegar is only an acid of about a pH of 6, but if a pH plunge has been an issue, leave that stuff out of the process. :)

When I was a year or two younger than you, my parents gave me my first 10-gallon tank (to go with several bowls purchased with paper-route earnings). Nobody knew much of the nitrogen cycle back then. At age 13 that 10 crashed, I scrubbed everything out and it bit like you, lost all but one pregnant female guppy. In a year she had become 232 common guppies!

The over populated tank was in trouble again. I asked my mom if I could wiggle out of selected after school obligations that day (no paper route anymore) and try to save the tank. She agreed. The gazillion guppies were removed to three bowls (sizing the guppies that way). The water was all drained, the gravel in the tank was rinsed of obvious dirt outside with the hose.

Those were the good old days where well water in Chicago's western suburbs didn't need to be chlorinated. The good ol' finger thermometer allowed me to refill that 10 to roughly the temperature of the guppy bowls - which also held
a pretty good collection of transient Valisnaria and hornwort.

The Val was replanted (at least by 14 I knew to poke a hole with a finger, gently place the plant roots in the hole and carefully push the gravel in on top). Then the hormwort was returned, the box filter (having been rinsed and the carbon stuff replaced) and heater were put into place. The heater and air pump were plugged in. The light was left off that evening.

The fish were also gently returned and except
for the loss of 2 of the ever delicate neon tetras, there were no losses.

Looking back, the relative success involved getting rid of all of the nasty water in the tank, not letting anything dry out but everything was well rinsed, all the bad water was replaced with reasonably safe stuff and we had a lot of fast growing (and heavy feeding) plants. Not much was fed to those fish the first week.

I miss having fish-safe tap water like that. ;)

So helpguppygirl, I think you know how to give your survivors the TLC needed to more than repopulate your tank. It is not unusual for all
of us to have a disaster or more when getting into the hobby. Because "life stuff" happens we will also find that periodically we haven't time to do what we need to do with our aquarium(s).

The trick is to learn to anticipate and avoid those disasters. But <sigh> they may never entirely go away. :)