Poor Mans Fertilizer and CO2 system

Hi,

i have a question regarding fertilizing plants and CO2.

I found an article on making your own mixture of K2SO4, KNO3, MgSO4
and trace elements to fertilize plants.
(http://www.cam.org/~tomlins/algae.html)
Now, if you use tap water, it will already contain most of the above so if
you
add such a mix, wouldn't that bring to many nutrients into the water?
Or are potassium and magnesium pretty rare in tap water?

Also, if you start using this DIY fertilizer, are there testkits to measure
those elements or do you just go by trial and error?

The reason i want to try this is that i'm having a slight increase in algae
growth
(hair algae). I use a liquid fertilizer but i don't think it's adding to
much elements
that are also usefull for algae (sera florena). But on the other hand, i'm
not sure
it contains potassium which seems like a must have for aquarium plants and
to
fight algae.

On CO2: my CO2 levels are way to low (+-1.2 ppm, PH 8, KH 5)
so i'm also going to build a DIY CO2 system. It seems easy enough.
Anyway, if you buy one of these systems, what would be the initial cost
and how much do pay to keep it going?

Regards,
Benedict