Clown Pleco Behaviour

[forever]

Hi All,
Currently have 4 clown pleco, one of them has taken to removing all the sand from one corner of my tank under some bog wood, so that its just down to the bottom glass, i replace the sand and the next morning its all moved again!! Any clues as to why its doing this?

thx

 

[techen]

My pleco moves the sand all the time, He'll go upside down on the wood and then flick his tail madly which forces all the sand to push away under the wood. I just leave it as he does it all the time