Mayfly Larvae in tank

[smileandnod]

I'm not sure if this is the right place to post this, but I have just found two mayfly larvae in my Apisto tank.

I have no idea how they could of got in there, and I don't know i I should be having a good hard look for anymore, in case they are harmful.

I have added nothing new to the tank recently, most of the water I use is RO, so I can't imagine where they have come from.

Ian.