Amount of Ca and Mg for my tank

Here's a related question: If my tap water has 20ppm Ca and 1.6ppm Magnesium on average, do I need to dose GH booster?

I've got a 20gallon with fluorite substrate, pressurized CO2 (keeps my drop checker a steady green) and 130 W of light over it. Plants are growing great. So's the algae *sigh*. I'm struggling with green dust algae and the dreaded clado. I'm guessing it's from fluctuating nutrient levels - I haven't been doing proper EI dosing. I've just been throwing the ferts in once per week, after the water change. I'm beyond tired of testing the water, so I'm thinking I'll adopt the EI method, and just dose every other day and do that big water change once per week.

I'm rambling...

Anyhow, is that enough Ca and Mg? If it's not enough of the latter (which I suspect), can the plants pull the Ca from the tap water, and I can top up the Mg with epsom salts?

Other stats on the tank:
pH is around 7.3
using a Magnum HOT filter (love it!) rated 350gph flow. I've slowed it down a bit by putting a sponge over the intake
substrate is fluorite as mentioned, mixed with geosystems gravel (same shape and color, just a sort of inert clay gravel)
hardscape is some natural river wood and river rock (which I boiled the heck out of!) from the Fraser River.
Tank temp is room temperature, about 74F
CO2 is running 24/7 at about 2bps (drop checker is green to yellow-green)
plants are a bit of this and that: ludwiga, rotala, cabomba, some crypts (including parva), some vals, hairgrass, a nice red plant who's name I forget, and some odd and end stems I pulled out of my 90 when I switched it to low light
fauna is a pair of GBR, a bunch of platies, some cherry barbs, panda cories, couple of otos, a bamboo shrimp, a handful of tiger shrimp and 8 amanos.

 

If you cut the light on that 20 gallon tank from 130 watts to 65 watts - remove one bulb - your algae problems will be a lot easier to solve. If you raise the light fixture a few inches above the tank, it gets easier still. You really have far too much light now. Then switching to the EI method will do a lot of good, as would making sure you have good water circulation all over the tank - all of the leaves should be in motion.

 

the lights are mounted about 7" above the water's surface (it's an open top, rimless tank). And the tank is more like 22G (it's a funny size), not that it matters much. 6.5 wpg is high light, I know, but I didn't think it was too high? If I drop it down to 65W, that's just 3wpg, which is only medium light I thought? The tank doesn't look unnaturally bright - it looks quite nice, IMO. Except that I'm having to wipe down the glass every couple of days.

 

IMO, 65 watts over a tank that size should be able to grow about anything quite well. Mind your CO2, ferts and water changes and it'll grow in just as good as it would with twice the lighting.

 

I should've clarified - I've only got the full wattage on for a 3 hour burst. For the remaining 5 hours of my 8 hour photo period, it's only 65W.