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Green water and GH

Hello all.

Something interesting occurred this week in three tanks
containing green water. I have four tanks all moderately
well lit. My tap water is GH<2.5.

The four tanks:

15 gal, bare, three SAEs, some plants, used to supply green
water to the following two 5 gallon daphnia setups

two 5 gal, bare, daphnia, aerated with large bubbles, one
tank is partly shaded

5 gal, sand over soil substrate, some plants, many guppies,
sand is somewhat alkaline, used as a green water backup

Three days ago, concerned that the daphnia might lack the
calcium to grow, I added about a 1/4 tsp. of powdered
dolomite to their two tanks and twice that amount to the 15
gallon. As of yesterday all three tanks had virtually cleared.
The 15 gal had been pea soup for six weeks. Usually the
daphnia in the 5's took several days before requiring a
refill from this tank. The guppy tank did not receive any
dolomite and remained very green.

Usually, but not always, I associate green water with zero
nitrogen. Perhaps the cause of green water is any input
deficiency that affects plant growth. In the past pH doesn't
seem to have affected the green water, and the guppy tank
has some carbonate hardness. I can understand that the
daphnia may be growing better in the 5's, but the suddenness
of the 15 gallon clearup surprises me. I have noticed over
the years that green water tends to eliminate other algae
forms and plant growth flourishes when it clears.

Dave Whittaker
Gloucester, Ontario
ac554 at FreeNet_Carleton.ca