The main reason for the algae in the UAT is because most people fill the main tank from the UAT so any contaminates in the fuel bowser, water ect are pumped straight into the UAT, any filter in the main tank is useless because the filling is on the wrong side of the filter, the algae is formed on water that lays in the bottom of the UAT, normally called a water bottom, the best way of avoiding it is to have a seperate fill in the main tank and use it to drain the tank and UAT at the end of a flying session.

The water in the main bowser is from condensation forming inside the cannister, get hold of a Mr Funnel filter which will pass kero, but not water and filter your bowser if its been layed up for a time.

Mike