

My guppy fry are dying
Hello, I've had guppy fry for nearly a month now. They were doing fine, but for the past few days, they've started dying off one by one. As you can see, I have a large aquarium with other fish that are all doing very well. I've placed the fry in a strainer to protect them from predators in the tank. I should mention that it's positioned right next to the filter so it gets some flow-avoiding stagnant water-but not too much current.
When they started dying a week ago-I'm up to the seventh death today -the strainer was covered in diatoms (brown algae). I cleaned it, but the daily deaths continued. So, I performed a ~60% water change on the tank (300L) and cleaned the filter at the same time (using tank water, of course). I put them back in the strainer and shifted it slightly more into the pump's flow to improve circulation; however, I was worried about the current being too strong and exhausting them to death, so I didn't move it too far (that was two days ago).
Today, I shifted the strainer a bit more because I felt it wasn't quite positioned right relative to the filter's flow. I also forgot to mention that I regularly check for leftover food at the bottom of the strainer and siphon it out with a manual vacuum if necessary, though I do leave a little bit so they can graze if they need to.
Does anyone know what might be causing these daily deaths?