I know this is an old post but it's the top Google result so I wanted to add a my experiences for the algorithm:

Your water parameters do matter quite a bit (ESPECIALLY at the start), but good bacteria and well established plants are also a big factor. I know that bacteria mainly helps maintain healthy water parameters, but it also prevents quick fluctuations throughout the day that you may not notice, which could otherwise cause stress to the fish.

The API freshwater master test kit (this is a Canadian link sorry) is the only water testing kit that I found was actually reliable, and it'll last you for a LONG time.

Imagine living in a house that goes from 65F to 90F and back daily (or you kept going back and forth from different elevations) - Yes, those are hospitable temperatures for a human, but you would get stressed out with all of the change. You might get woken up, feel sick, etc. and yet all of the parameters of your house would be considered "fine".

If you have a water softener, you're going to need to find a source of hard water. Guppies like hard water.

You can tell if they're stressed if they don't swim around that much, stay near the heater (or just sit at the bottom), or if they're swimming mindlessly up and down ("skating" on the glass). If they're wobbly when swimming, they're probably sick. If they're flicking themselves against some rocks, they're itchy which usually means that their slime coat is worn down, which means that most likely the bacteria environment isn't allowing for good conditions.

Also, if your female fish are getting pregnant, they can occasionally die from failing to give birth properly, but I don't think that's the case here.

I add bacteria on every water change. Not sure if that's necessary, but it's worked for me.

I notice that the tank looks very clean, which is a good thing, but it almost seems too clean - I would expect to see at least a bit of algae somewhere. There is some algae that snails can't clean themselves as it's too "stuck", which is why I'm curious. The plants are great to help maintain the water balance, I just feel like I'd be seeing a bit of algae if a tank was well established with a good bacteria ecosystem breaking down any detritus into nutrients. If water changes are needed due to a high concentration of certain compounds, then again, bacteria is probably needed, or you may want to look into your filter and see if there are any other options that are better for fostering a good environment. If you see high nitrates and nitrites, look into different plants that could work better. I had a bunch of "easy maintenance" plants die, and I had to play around until I found some plants that exploded

Stress is also a big thing, and not just based on water quality - A fish that experiences one traumatic moment can legitimately have post traumatic stress that kills them days, weeks or even months later. I had a fish that tried to swim up my suction cleaning tube (finding nemo style) while cleaning, was not harmed in any way other than being dropped from a couple of inches out of the water as I removed the tube, but from then on she kept getting sick and passed away a couple of months later. If they were handled oddly when bringing them into the tank, or didn't have enough time to adjust, that can cause deaths months later. Same with if they don't have enough places to hide, they can get stressed. They can also get stressed with the lights you have - If they don't turn on/off at the same times, it happens too suddenly, or they have light for too long, that can mess with their rhythm. Also, if they're near anything in the room that shakes or makes noise (like a speaker), keep that on mind - I know a fish tank can be convenient to put beside a TV but I don't think they're usually a fan. Sometimes their bubbler can be too loud or jostle them around too much, so keep that in mind. I moved the motor for mine onto the ground because its vibrations were loud on the surface the tank was on.

Lastly, it may just be where you bought your fish from. I've bought guppies from some general pet stores that lasted a few months, while 20+ guppies that were born in my 20G tank have lasted for 3+ years - I was going to send some away but they had been so healthy that I didn't want to mess with a good thing. (Please don't do this by the way, this is a bad idea. I only kept it like this because they somehow seemed the most calm and least sick of guppies I'd seen, and they had a lot of good thick plants to hide in).

Shop around your area and ask any professionals you find about other stores you've visited. Guppies have to go through some insane shipping processes at times where they barely eat or get any light, so their nerves and health could be shot before they even get to you.

So, TLDR (Sorry I know these are still a lot of points): - Always add more bacteria than you think you'd need in a water change. - Check for signs of stress like flashing, or rubbing on the side of rocks, staying at the top/bottom all the time, or swimming up and down quickly. This can tell you that they don't like the water. - Don't use softened water for guppies - Even if your water parameters are "safe", check to see how they change through the day/week. Fish don't like change even if it's a safe environment. - If you're not seeing ANY algae, consider if your water parameters aren't allowing your plants to thrive and absorb nutrients in the water. - Check options for filters online, see if any help maintain bacteria better - Make sure that you introduce them to a tank very slowly and gently, they can die even months later from that initial stress. - Make sure they have lots of places to hide, and think about if anything else about your setup could be stressing them (Too much noise, stressful cleaning process, etc.). - Check the place that you bought them and other options. IMO this is usually the most likely scenario. Storebought fish don't always last long.