IME algae likes:
-nitrate
-phosphate
-light
Try reducing the first 2 via water changes, or a carbon source such as vodka or vinegar..it's a small tank so definitely go very slow, and do your research on carbon dosing..plenty of info here on r2r. Carbon is best for nitrates, so you could look at media such as phosban or similar for reducing phosphates. You could also try a refugium with some type of macro algae such as chaetomorpha that will feed on no3/po4.
Before any of this test your nitrate and phosphate levels, so you can track changes.
As far as a DI chamber, I am able to get a 0 tds reading without one. Check out
Amazon for a TDS meter, they are less than $20. This will tell you if you need DI. You are not running your lights very long, so I do not believe this is your issue. It would also be helpful to know what you have in the tank as far as livestock.
You could also add some marinepure or matrix, more beneficial bacteria never hurts, especially the anaerobic kind that grow in very porous media/rock.
Again, you have a small tank so water changes alone may be easiest. Hope this helps