[hoppycalif]

First, I would not use aragonite or pea gravel in the substrate. For a tank that size I would buy a bag of black Flourite Sand and use that.

I would use tap water instead of RO water.

I would use Excel as a carbon source, and dose KNO3 and KH2PO4 plus a trace element mix, such as Flourish to feed the plants.

The reason you had problems with blue green algae is the lack of nitrates in the water.

One 32 watt PC, if it has a bulb with color temperature between about 5500K and 10,000K, will be adequate for an Excel tank.

 

[hoppycalif]

The most effective step in avoiding algae is having healthy growing plants, and a lot of them in the tank. That is one reason for fertilizing - to get healthy growing plants. What heavy metals do you think are in your tap water? I assume you drink the tap water?

 

[hoppycalif]

For a 6 gallon tank you could probably afford to use RO/DI water, even buying it. Then you could add Equilibrium to the water to bring the GH back up to where it would be good for both fish and plants. I wouldn't worry about the pH, if it were me. Just go for a GH around 6 or so dGH.

You will still need to dose NPK and traces to grow plants better than algae.