He spotted the pleco on the glass and was curious
thats adorable. you should get him a lil chair to sit and watch your fish.
I used to have a 20L with african cichlids. My cat would lay on top of the glass and watch the fish for hours. (I had to cover all the spots which had access to water because he was trying to catch them)
He eventually stopped one day when the versa-top broke and he fell into the water. Now he watches the fish from a distance.
I've tried sitting him in the chair and holding him in front of it he seems to only do it when no ones looking at him. I'm glad I got the pic tho he was only there for a couple seconds