Hello everyone, first time posting here.

So, I've had an aquarium for I think 2 years now with weird results. I'm here to see if there's an explanation.

I did the prep thing where you cycle the water without fish, introduced a few plants (idk which ones anymore), and even tossed in a little water (bubble) wheele. The first fish I got were a school of tetras and maybe a guppy or two. The tetras, unfortunately, but predictably, died shortly after getting them, so that was money flushed down the toilet. IIRC, the guppies lasted a few weeks, I introduced more plants, and eventually, another 2 guppies. At some point, a few couple died. I went to petsmart and to an acquaintance about my problem, and they said it could be the nitrates/nitrites/ammonia. I didn't fully know about it at the time, but after doing some research, I got the hang of it.

Queue to the last 4 fish I've owned. The first of the 4 died months ago. He was a black guppy that survived while the other 4 that were with him died. I had only him for a few weeks before I decided to get 2 tequila sunrises. The 3 cohabited wonderfully for about 2 months. One day, I noticed the black guppy's swimming was weird. The next he's blending into the filter's filter. I'd say within 2 weeks, one of the tequilas copies the first guppy.

At this point, I'm down to 1 and decide not to get anymore as I don't want to 1 keep spending money and 2 see the little dudes ☠️. I have him for over half a year, just vibing alone. He and my finger became good friends (through the glass), and all was good.

Then I got another guppy ( I can't remember the specific type. Just know its tail has a cool leopard print. Looks similar to the tequila). Everything is fine for like, 2 weeks. About 3 days ago, I noticed the tequila's swimming is weak when I leave for work. He's dead when I return. Now I'm down to my newest, and I'm so confused as to why I always end up with just one survivor. Does anyone have an explanation?

TLDR: New aquarium owner fucks up at first but then learns. Eventually, he can maintain fish alive. However, it goes from 5 to 1. 2 fish are introduced, of the 3 only 1 of the new survives. Months go by, and a new fish is bought. Now, the new fish is the last survivor.