If you use a bucket to collect the old water, just wash any sand you suck up and put it back in the tank.
Years ago I had a small Clearseal tank which came with a black plastic lid that couldn't fit a light inside. It was a quarantine tank, so not having a light didn't matter. Then I decided to use it as a betta tank so it needed a light. My local DIY shop sold plastic panes for greenhouses and they cut one down for me. This did not bend with heat but did get very scratched over the years.
Fast forward a few years and I replaced the Clearseal tank with a new one. This had a glass lid but it only covered half the tank so I went back to the DIY shop and bought more plastic to make a lid. They told me this time that it was perspex. However this lid did bend.
I don't know what the first greenhouse pane was made of but presumably it was not perspex. Google suggests that greenhouse panes can be made from perspex (acrylic), polycarbonate and PETG. Maybe try and source polycarbonate rather than perspex for a lid?
Years ago I had a small Clearseal tank which came with a black plastic lid that couldn't fit a light inside. It was a quarantine tank, so not having a light didn't matter. Then I decided to use it as a betta tank so it needed a light. My local DIY shop sold plastic panes for greenhouses and they cut one down for me. This did not bend with heat but did get very scratched over the years.
Fast forward a few years and I replaced the Clearseal tank with a new one. This had a glass lid but it only covered half the tank so I went back to the DIY shop and bought more plastic to make a lid. They told me this time that it was perspex. However this lid did bend.
I don't know what the first greenhouse pane was made of but presumably it was not perspex. Google suggests that greenhouse panes can be made from perspex (acrylic), polycarbonate and PETG. Maybe try and source polycarbonate rather than perspex for a lid?