My 6' low tech planted tank has been set up for eight weeks now and in the last week it has started to develop a brown surface scum that keeps building up. Today I cleared it by floating newspaper but by this evening it was coming back again. I am using a Tetratec Ex 1200 cannister filter and noticed that the minimal surface current was not reaching the end of the tank so I removed a mass of Hornwort and cut back the giant Vallis leaves. The problem seemed to start when I introduced 20 Cherry red Shrimps and put a large sponge filter over the end to stop them disappearing into the filter. Am I right in thinking that increasing surface flow would be counter productive in a low tec tank? Also adding Mollies would probably be the end of the shrimp? I am going away shortly and really need to sort this out quickly as I cannot ask my fish sitter to float newspaper every couple of days. So far I have not done any water changes but would do so if you think it would help.