Well leaving the filter unchecked for quite some time could be a dilemma for a breeding ground of algae causes. Did encounter this situation previously.
First- total black-out (no lights) for 4 consecutive days (wrapped newspaper around the tank).Results: algae diminish including the cyanobacters.
Second- Introduced the clean-up crew. (7 Flying foxes and 2 SAE's, all juvenile). Results: The crew loved the algae residues.
Third- 3% H2O2 sprayed directly to affected parts, (even used 10 ml syringe). Results: did eliminate some stubborn algae on the affected plants
Fourth- Regularly added 10 ml DIY 1.5% glutaraldehyde. Cleaned the glasses with magnets and plastic scrub pads dabbed with 1.5%GA during the water change. Result: Take much longer for the brown film to develop on glass surface.
Fifth- Remove dead and dying leaves. Better to have a stem without leaves than to have leaves that will promote algal growth (lesson from Diana W.) Result:Minimized possible hosts from unsterilized plants and as well causes of the nutrient imbalance.
Last not but not the least. Do a filter check. My average of 2 hours 55 mins. of watching the tank and 5 mins. of checking the filter completes my day. BTW, I'm only home during the weekend. Weekly water change of course is routine but 8 to 40 gallons is something that doesn't bother me. E.I. with Diana W. principles help me control some of the situations in my 90 G. Thanks to my daughter and wifey for the scheduled 1.5% GA's, feeding, traces, and NPKs addition.
Hope this help, just do research first.
General Result: NO FISH DIED DURING THESE PROCESSES, yeba!!!