Use a syringe to inject 3% H2O2 directly on the algae. If you have potted plants, take them out of the tank, squirt H2O2 on the algae covered leaves, wait 2-3 minutes, then put the plant back in the tank.
Like Tom said, cherry shrimp will completely clean that black-brown algae in a matter of hours. I have them in all my planted tanks.
Sounds like your light and water parameters are ok, but you are right in that you need to test for phosphate. That may be a problem. Your heavy feeding schedule is also adding a lot of nutrients to the water column that is feeding the algae. Larger water changes would help with that.
If erythromycin worked it was not algae. If it had a foul odor, it was cyanobacteria. You'll never forget the smell once you smell it. When I had a planted discus tank the water changes were too great to keep the right/correct amount of nutrients in my water for a lush green tank. 75% or more every day or every other day. I had to use the EI method for nutrients. I bought all my chemicals from a hydroponic shop in raw form and made my own mix to make it cheaper because of the large WC's. Really is difficult to buy nitrate in most stores here because of it being used in explosives. That method worked great for me. I got rid of hair algae on anubias plants recently by dipping them in H2O2, which is a very low tech planted tank. I've also used the syringe method like mentioned above. I have had SAE that even ate the cyanobacteria; I couldn't believe but I saw it myself.
So, an update on where I am. Apologies for tardy reply, wanted to have some, hopefully, positive news. Firstly many thanks to all that contributed to my query, some really useful advice. I implemented the following measures.
1. Introduced a strict 8 hour continuous lighting regime, 2.00pm to 10.00pm daily
2. Removed as many plants from the tank as possible and sprayed them directly with H2O2
3. Purchased 2 litres of Seachem Excel and dose 12.5ml in my 500 litre tank (1ml/40 litres) every second day
4. Installed a solid navy blue background to the side and back of my tank, these restrict the amount of natural light entering the tank, also improved the look of the tank
5. Moved some of my anubias into deeper areas of the tank, further from the lights
6. Increased the amount of water changes to 40% every 3 days
7. Decided not to go with Ottos or SAE, frightened of these fish sucking onto the sides of my discus, I know it doesn’t always happen, but don’t want to risk it.
It is early days but there has been a big improvement in the plant health and the algae is definitely reducing. My white pots were previously covered in algae and this has almost totally disappeared. Even algae on the glass, whilst still appearing, has reduced. Some photos are attached to show improvement, not perfect yet, but vast improvement, sorry don't know why photos are all sideways. Thanks once again.
Sorry, just trying to see if I could give the photos the correct orientation, problem solved
Plants look great as do your discus!! Keep up the good work. As to the SAE and ottos they will work fine in your tank. They are in all of mine. Qt them before adding to your discus. SAE do not have a sucker mouth so no fear of eating slime coat. Ottos on the other hand may but being so small the discus just shake them off. 31EF4056-9468-4441-B622-716EEB3AF11E.jpgsae on either side of tank.A3040D19-1833-452E-B504-BF5B0B2F53B6.jpg my other tank and there right in the middle with the discus.
Wow .. your plants and especially swords look way better now. I echo Tom: ottos and sae are just fine.