I'm very confused. API algaefix SAVED my fish??

[TheFaceguy]

I'd like to start off by saying I think the product is kinda awful, and I really wouldn't have used it at all if not for the info below, and I probably wouldn't recommend it even after my weird success. Anyways, here's my dumb mystery story, maybe somebody can explain to me what has happened.

A few months back I started a new tank, 40G breeder, got it all cycled, added some fish. The tank some some neon black tetras, some danios, some otos, and some cherry barbs. Things were alright for about a week, but soon I started seeing flashing, the danios lost their 'zippiness', the cherry barbs became increasingly shy and thin, the otos became lethargic and the tetras... well just just continued doing their own thing, I guess.

So I assumed a water quality issue. I tested 2 times a day and saw no issues (PH 6.8, ammonia 0, nitiries 0, nitrates under 20ppm, no GH or KH issues outside of the species ranges). Despite this, I started to change 20-30% of the water every other day. No change. Eventually I started losing danios, which are supposed to be pretty hardy. They started thinning rapidly, and so now I assuming internal parasites. I used pazipro to the specifications, removed my purigen (not running carbon). Nothing. More dead danios, cherry barbs barely holding on. I tried treating using some other parasite meds, the Fritz one, and API general cure. No change, still losing danios. I tried treating for bacterial issues with maracyn, tried ICH-X, even tried melafix (though I think its snake oil). Throughout all of this, 9/12 of the danios had died, 2/8 of the cherry barbs died, 2/8 otos died. Most of them died when I WASN'T treating for anything, so I really don't think it was the meds, and betweed each treatment gave them 1.5-2 weeks to 'rest' before the next one. I was at a loss. I stopped medicating, deciding to just... let nature take its course. A month or so passes, and fish just keep randomly dying. They were so unhappy, and I had no idea how to help...

Well, the holidays rolled around and despite my regular maintenance, this tank was my only tank in horrible shape. It's also the one in the living room, and like an idiot I dosed too much ferts for the plants right before the week when people were coming over, so now on top of my pathetic sickly fish I have to watch die every other day, it has an algae problem! Great! The otos weren't helping even slightly, they were barely clinging to life. At wits end, rather than manual removal, I decided to quick fix it and chance some algaefix despite the fact that it has been known to in some cases kill fish. I half dosed it for the first time just to be extra safe, but braced for the worst...

5 minutes later...

The remaining danios were dashing around, and active. The cherry barbs came out of hiding. The otos started eating the algae. Like magic, the tank came alive, not from fish stress, but from them actually acting the way the species are supposed to! So, I feed them of course, because they were so thin and weak and they ACTUALLY ATE A REASONABLE AMOUNT OF FOOD FOR ONCE! Their activity kept up for the next 3 days, when I dosed a full dose, as per the instructions, and they didn't seem to mind, and kept being active. Now we're 2 weeks past my last dose of it, and the fish are as happy as can be! They're still active, the danios even started doing mating dances and spawning. I can not believe it.

So this entire story is a long preamble to the big question... what the HELL happened here?! What on earth was wrong with this tank that an algaecide was the ONLY thing to actually fix it? I know of nothing that could POSSIBLY cause these issues and be 'cured' by using it. If anyone has any idea of what happened here, I would love to know, because I've never seen anything like it in years of fishkeeping. I'm truly at a loss.

BONUS STORY: the algaefix ironically didn't even actually kill the algae. It just made the otos healthy, and since I only overdosed ferts once, nature and the newly reborn otos fixed it

[Delapool]

Hmmm, interesting! Does it say what’s in the algaecide (too slack to look it up).

Have you done any water changes since?

Speed of recovery suggests it ‘fixed’ a water chemistry problem (plausible??), but I can’t figure that out since you have multiple tanks I think.

[TheFaceguy]

The bottle lists:

• Poly[oxyethylene(dimethyliminio)ethylene(dimethyliminio )ethylene dichloride (4.5%)
• OTHER (95.5%)

And yes, there have been several water changes since. No other tank has had these issues. The bottle says "acts fast" and the reviews for the product saying it killed all of their fish often say it happened VERY quickly, within 5-10 minutes. So my only theory is that it killed SOMETHING that was stressing them.

[Delapool]

Ah, missed the flashing. Was that all the time along body and not just on gills after feeding, etc.

I guess if it can bump off fish - parasites could be possible. I’ve not used it for few years as fish were fine but plants weren’t and algae was undecided.

[TheFaceguy]

Flashing was somewhat common and happened randomly throughout the day, not after feeding, and usually the belly and sides. At one point a cherry barb that flashed a lot went down to the sand and literally rolled in it, constantly, for about 2 minutes. Never seen flashing like that before. I treated for parasites 3 times with three different meds (api general cure, maracyn and pazipro) thinking flashing was external parasites and wasting away was probably internal, but that didn't seem to affect it at all, and seeing as the water params were constantly good... I have no explaination.

[Delapool]

Huh - well I’ve learnt something new. Looked it up quickly but didn’t ring any bells from any other threads. Good news it worked anyways.

[TheFaceguy]

Thanks! I was hoping somebody might know what caused this success, as I didn't find any info elsewhere. Looks like next to no one has had a similar experience, sadly.