Hi Matt,
Do you have your nitrate reading at hand? Since plans (which includes algae as well) use nitrate as well, excess nitrate could cause algae blooms - too much nutrients for the other plants to consume and thus causing a breeding ground for algae.
You can try adding Marimo Moss Balls which should outcomptete the algae for the nutrients in the water.
You can also use something like Nutrafin Waste Control added with your weekly water change to help with the breakdown of organic matter in the tank.
As a matter of interest, do you keep your CO2 unit running all day? If so, turn it off at night as your plants only require CO2 during the day.
I would ignore this advice as per the following:
Nutrients do not cause algae. algae does use nutrients but only if the plants are not growing. Therefore excess nutrient will not be accessed by algae unless there is a defficiency of another nutrient like phosphate, potassium etc.
No point running CO2 in your tank because it IS only needed by tanks with higher light. The more light a tank has then the faster photosynthesis is driven and therefore the CO2 in the water and nutrient supplied by fishwaste is not enough for this faster photosynthesis. This is why people who use higher light go on to CO2 and fertiliser dosing.
Moss Balls are not moss. they are in fact......algae so I can't see the point in putting algae into the tank in the hope of getting rid of algae!!!
I would check your wattage against your tank volume. You have a 20USG tank and therefore need approx 20W of light for the plants you have.
My guess would be that the algae is appearing due to the crypts. Great plants and the majority of the tank in the right picture of my signature is made up of crypts BUT....when crypts are first put into your tank they tend to adapt badly to the change in parameters. They will then go through a 'melt' which means that their leaves dissintegrate. After which they will then recover and grow new leaves which will then go on to give you beautiful plants.
The reason this is a problem is because when this happens to any plant not just crypts that deteriating leaves leach ammonia into the water column and THIS is what causes the algae. You may find that the black algae you have is only on leaves that are deteriating. Most people assume the algae is killing the leaves as they see the algae then the plant 'dieing' but this is because the algae grows the minute ammonia starts leaching and this is normally when there aren't many visible signs YET of the plants problems.
Therefore I would perform a decent water change daily until the crypts recover to remove this ammonia. Chop off all the affected leaves upon sight and ditch the CO2 until everything is recovered.
Check your light and if needed upgrade to 1WPG.
Once the plants have started growing again and look nice then you can decide if you want to return the CO2 to the tank. If so you may need to dose some fertiliser but that is a different story.
Picture of the tank would help just to see how many plants you actually have.
Andy
