Do you have a short-list? I have three of the Interpet ones like
this. (Technically, I only have two, because I have one long hose [15 meter, 19mm/3/4" inside diameter] that I drain most tanks with, and on that I swap the actual "nozzle + pipe" section, so that I have a long vac for the big RIO 400 tank, and a shorter one for the smaller tanks).
The third (or second) one is the original small one that I bought for my 96 liter tank. It has a smaller diameter hose and is useful when doing the small tanks, or for filling a bucket of tank-water when cleaning filters for example.
There are more advanced electric ones. I've never used those.
My father-in-law had one of
these, and when I tried using it, I never got it to work as I expected. Could be my fault - I never tried very hard to get it to work.
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Mats