[MatPat]

Your algae problem is probably due to the Nitrate (NO3) reading of zero. Plants need NO3, and also PO4 (phosphate) along with carbon (CO2) and Trace elements (iron, etc). The key is to not run ot of any of these.

It also seems you have some fast growing stem plants in your tank. Sag, and your 3 hygro species all grow pretty fast and need the above nutrient to continue growing. When you limit one of the above, NO3 in this case, the plant growth usually slows and the algae growth increases.

 

[MatPat]

Sorry HOC, I missed your question. Trena's suggestion are good ones. You can also purchase Spectracide brand stump remover for your KNO3. As noted, it does contain potassium (K) along with the nitrate (NO3).

You can use liquid ferts such as Seachem's Nitrogen but it is much more expensive that way. One lb of KNO3 runs $3-5 (either stump remover or from Greg Watson) and lasts me about a year dosing both a 75g and a 30g. Shipping from Greg could be a concern but if you make one big purchase, the saving including shipping would be way cheaper than using purchsed liquid ferts. Whichever method you use, liquid or dry, try to get individual ingredients versus a comprehensive supplement. That way you can add only what you need.