GSA at PO4-8,4ppm???


Guru Class Expert
May 18, 2010
Hallo! My name is Luka , i'm coming from Croatia.
I have a Rio180 lit and i'm trying to make a dutch scape..
(Please excuse my english)

Some technicalities:
Light- 2x45wt5 , 2x30wt8
Fotoperiod: t8- 10h , t5-5h (t8 , t5 t8 , t8)
Filtration: 2xinternal filter (2x600lit-h)
Flow pumps: Atman 2x500lit-h , 1x350lit-h
Overall flow: aprox 2500lit-h
CO2: 20kg pressurised tank , fabco nv
CO2 level: DC is yellow , CO2 goes through an airstone and into the three Atman flow pumps and spreads across the aquarium , co2 is set as high as it goes , if i turn it up higher my fish start to pass away.

I've had dome potassium deficencies in the recent past, after increasing potassium there was nitrogen and then po4 deficencies..
Last few weeks i increased the dosages pretty much:
NO3- 36ppm (all doses are weekly)
K - 60ppm
PO4- 8,4ppm
Plantex csm+b - dosed so that Fe is 1,7 ppm weekly

The problem is, Few days ago i cleaned the glass that had GSA on it since my po4 deficency problem, and this morning i discovered GSA started to appear again..
Is it posible that there is so much plant in the aquarium that 8,4ppm of PO4 is not enough?
How much po4 can i put in before it is too much?
Would it be ok to increase po4 some more and do 70% weekly water changes just to be safe from overloading?

Sorry for the poor pitcure quality, i just took the pitcure few minutes ago with my phone..
Last edited by a moderator:

Tom Barr

Staff member
Jan 23, 2005
If you have GSA, it's either CO2 or PO4.

If you rule out PO4 by dosing and good sized water changes, then there's likely only a CO2 issue.
You also have high light, I'd say you only need the 2x 45 W T5 lights for 8-10 hours.
I do not think anyone has over loaded on PO4, I add 2-4X as much as you do.

Since I add a lot, I know the only other reason I might see glass algae is due to too much light and not enough CO2.
I add enough current to cause the water to slightly ripple, but not break the surface.

This will help add O2, so the fish are having an easier time and the CO2 is more manageable.
Also, with less light, you have less CO2 demand, so it's easier to add without harm to fish.

More light is not better.

Tom Barr


Guru Class Expert
May 18, 2010
Thanx for answering!
I will cut the lights as you suggested, only t5 9 hrs-day.
Also, i will increase PO4 a bit so i can rule it out as a cause if GSA continues.