I am in the process of setting up two super low tech tanks. Ultimately, I want the inhabitants of the tanks to be the source of ferts for my plants, but initially, I want to give my plants a boost to get going prior to putting the fish in.

I've had a couple thoughts.

1 - Water from inhabited fish tanks. When I do water changes, just swap the water from the new tank and the inhabited ones.

2 - Miracle-Gro Houseplant Food. I have a bottle of the liquid stuff that I use on my houseplants. I read the label, but it doesn't provide much info on the inactive ingredients. Of course, I wouldn't use more than a couple drops at a time, and I would stop fertilizing at least one week before adding any fish.

3 - Commercial aquatic fertilizer. I'd rather not spend much, if any money on this, since I'll only be adding ferts once or twice.

Suggestions?

If you want low tech, meaning low light, you don't need to dose fertilizers. If you want to give the plants a boost, use DIY CO2, but just giving them CO2 for a short time can do more harm than good. Crypts, at least, may die back when you stop the CO2. The best way to do "low tech", in my opinion, is to use a fertile substrate, no fertilizers added to the water, low light, and just wait for the plants to adapt to the conditions and grow at the slow pace that is typical for low tech tanks.

If the tank is a small one, 20 gallons or less, you can use Flourish Excel to give the plants a boost, and continue using it to keep them growing at a steady rate. With larger tanks, Excel can begin to get expensive.

I would use some substrate tablets. Slow release, so as the other fertilizer sources build up the tablets are getting used up.

Another source is starting with good soil. This really is the basis for a low tech tank: Use the best materials to start with so the soil, fish food and water changes provide the plants with the things they need.