Sierra, I completely for got about the overflow always thinking...

Sierra, I completely for got about the overflow always thinking about guttering etc but when I spotted the offending rodent, which was quite large btw, the penny dropped and I remembered the overflow pipes which consist two 100 ml pipes which would present no challenge to a fully fit rascal to enter the tank.
Same thing in your case with the 50 ml pipe my guy could probably get in there if on Jenny Craig LOL.