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How did they stop working? Only time mine ever stopped is if I accidentally got substrate stuck in it. Blowing it out with a garden hose clears them out if that happens. What brand are you using?
The only other time I had mine stop was when the hose de-attached from the vaccuum. That is a really bad way to have your's stop working, let me tell you (especially if your not paying attention ).
I got them both working thanks to trying your ideas, it seem theres a plastic flap where the vacuum tube meets the hose and this was stuck hence no vacuum being created. Thank you