The friendliest place on the web for anyone with an interest in aquariums or fish keeping!
If you have answers, please help by responding to the unanswered posts.
You are better off with an external one that has no more than 3mm between the sleeve and the housing and also no more than 30 gph/watt for green water 20gph/watt for killing stuff and 10gph/watt for extreme killing of almost all bad stuff.