Hi Martin,
Sorry to read about your frustration.... For how long the 2 filters have been running on this tank? Are they well established and colonised? Might be a good idea to introduce some bacteria in them, to give the colony a boost. On water change, for how long do you keep the filters off? If you keep them off for longer than 30 min, the bacteria are very likely to suffocate and you can loose a considerable percentage of the colony.
Recently I have similar problem, I was using only one filter on my 60 cm, but as it is very heavily planted it was difficult to manage the bio load. To make things worst, we had a power cut off, because of a fault and the filter was off for more than 6 h, just couple of days after Introduction of the new fish and after a massive trim... In 2 days time, ammonia spike hit and I lost 70 % of the fish, brown algae covered all the plants, GSA covered the glass.. it was looking devastated. I was quick to install the second filter, but for the next 10 days....no improvement, despite the massive water changes. Then I have introduced some bacteria to the both filters and kept going with the regular water changes, high ferts, high CO2 etc. 10 days ago, I have decided to use H2O2 to clear the algae covering the plants. I have used a small spray bottle and sprayed the glass, the hardscape and over the plants(but plants were under the water level - I was spraying under water!!. If sprayed directly on plants when emmersed, most likely it will kill them!!). Waited about 5 minutes and then filled the tank back with fresh water. After few hours everything was still pearling and by the evening, all the algae were completely gone. Fish and shrimps were ok, didn't look stressed much. Next day, I did another massive water change to remove all the dead algae. Instantly I could see improvement in the plants, they were free of algae and were starting to grow again. I have used about 40 ml of H2O2 in 50 l tank.
Regarding the GSA - I sprayed them, but didn't remove them to see how they will behave. A week later they were still on the glass, but not growing at all and kind of brownish in colour. Last Saturday, I have scraped them off with a plastic card and they came off as a whole peace, completely dead. I use the H2O2 for spraying on the glassware as well and they are nice and clean, without the need of touching them.
Disclaimer: Now I would not advice use of any chemicals in the tank unless as very last resort, even though the H2O2 is basically Hydrogen and oxygen compound and is very mild. Always I would first try to remove the source of the problem, not the symptoms. I have decided to use H2O2, as I couldn't find a way to remove the algae from the plants (especially carpet and stems) and the algae were suffocating them - it was that bad.
My plants were not damaged after the use of H2O2, but I can't say if it will affect other species.
I have done some reading about the H2O2. No serious study has been made, but apparently it bonds with organic compounds or metal ions in order to release the O2. However one need to be careful as at certain( unknown for me) concentration, because of its chemical reactivity, it might become toxic(releasing heavy metal ions and other organic elements while breaking the compounds).
It also affects single cell organisms ... like algae and bacteria. Now I am not sure if it affects a lot the beneficial bacteria in the tank....it probably does, but from what I have read it is not very strong antibacterial product, so hopefully does not affect them much.
I think the plants will do it's job and once you get the balance sorted all ok.Good luck
