Nothing wrong with using an old aquarium for plants Tom. I have an old 10g tank that I have a load of baby Calatheas in.
I have a layer of gravel across the bottom which I keep wet, and a polycarbonate lid to keep it humid. The humidity level in that tank is 90% and my young calatheas love it.
Thing is, that tank isn't even damaged, it holds water, lol. It's even still got the thermometer strip on the outside, lol.
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Also, I went to my LFS over the weekend. I heard they were closing down so I went over to confirm it with staff. And they are! In June.
They had loads of empty salt encrusted 75g stock tanks from the marine side all stacked up for sale. All they wanted was £5 each!! I thought about what I could have done with a few of them in my plant/fish room, but then my thoughts quickly turned to what the wife would have done to me if i'd have taken a truck full home, lol.
So I left them!