Stubborn Brown Algae in seasoned tank

[Fenrir]

Currently have a 40 breeder with a stock list of:

2 Medium sized Angels 

2 baby Polypterus (don't start hating please only in there till my 210 gallon is done being scaped)

2 Kribensis 

2 SAE's

and a multitude of rams horn snails

Plant stocking:

pogostemon Helferi

Pogostemon Stellatus octopus

star grass

misc crypt species 

buce skeleton king

anubias gold coin

Pogostemon Decanseis

bacopa caroliniana/monnerie

Valisenaria Leopard

and ludwigia palustrus 

and my tank stats are:

PH: 8.2

temp: 79

Lighting: 2 fluval 3.0's at a photo period of 9 hours that ramp up to 85% then back down.

Nitrates: 40 to 80 (out of the tap it comes out at 20ppm)

Nitrites: 0

Ammonia: 0

With that out of the way I need some help problem solving. My tank is about 1 year old now granted the tank is only going to be up a few more months because it will be used as a sump for the 210 gallon I am setting up. That being said though I am lost to what is bringing on the Brown Diatom algae that is accumulating on the plants making them sad. I also have BBA but that has been slowly going away due to the SAE's. The snails have been going after the algae but not to an extent that eradicates it off the plants. My plan is to transfer the plants to the 210 so I want to get this under control before the transfer. What are some thoughts to help balance things out, I think it has something to do with Nitrate levels but not really sure how to handle that without R/O system which simply isn't in the cards. 

 

[Fish Folk]

If you can add some photos, that’s helpful.

I find this webpage on algae by Green Aqua to be an excellent reference: 

https://greenaqua.hu/en/alga-tajekoztato