Green water is the easiest algae to treat and the least harmful to our plants and fish... so don't stress Ivano
It normally makes an appearance in newly set up tanks and after major re-scaping or huge water changes on mature set ups.
Phosphates are what green water feeds on and I would recomend that you only dose iron, potassium nitrate and magnesium sulphate in half doses until it clears. DO NOT dose trace elements or potassium phosphate until you are all clear again!
Once the water has cleared you may use the full dose for everything but trace and phosphate which you can re-commence at a half dose.
I developed this method for treating "pea soup" from my own experience and I have since thought carefully of the reason why it has worked... This is just a theory, so I'm open to other suggestions/reasons/corrections:
In a balanced tank green water is consumed by free-swimming plankton, the population of tiny ciliates, rotifers and crustacea, especially copepods, that keep the algae population in balance.
In a new tank there is often an abundace of nutrients available that gives green water a chance to bloom... once the nutrients are depleted it takes the plankton/micro-fauna several days to increase in large enough numbers to consume all of the excess green water.
After treating a tank with medication, or after doing a major water change/re-scape in a mature tank we can actually devastate the plankton population, which allows the green water to make a "come-back".
For this reason I would advise against using a UV Sterliser as this will destroy the plankton as well as the green water, which means, as soon as you take out your UV Steriliser the green water will return.
If you want lasting results, I suggest you do a 50% water change (to remove excess phosphates), follow my modified fert regime (above) and feed your fish a minimum until it clears... then slowly increase the amount of phospahtes that you add so that the plankton which feeds on green water has an opportuntity to increase in population as the green water does in response to the extra nutrients available.
Good luck, and my apologies for the long post :oops:
PS: The cardinals do look great (nice and big) and they will be fine in the green water :wink: