"Shagster" wrote in message
...
I rented a Diatom a week ago that cleared the problem in just a few hours..
Of course, I returned the filter and now the problem is back.. A friend is
lending me a UV that's 6 years old, but the bulb is within a year old..
It's 25 Watts.. and yes, I've had it on 24x7.

Hmmm... UV is a sure-fire wire to get rid of green water, period. If it isn't
doing
that for you, there are a few possible causes:

- Something is wrong with your sterilizer, for example, the bulb is worn, or
the quartz sleeve or surface of the tube are dirty, so the UV light can't
actually irradiate the water. (Bulb life is typically 8000 hours with a
conventional ballast, or 12000 hours with an electronic ballast.)

- The wattage is too low (unlikely at 25W, unless your tank is larger
than around 200gal).

- The flow rate is too high, so the dwell time in the sterilizer too short
to effectively kill the algae.

- The flow rate is too low, so you kill everything that passes through
the sterilizer, but the sterilizer doesn't turn over your tank volume
frequently enough, so the rate at which the algae reproduce exceeds
the rate at which they are being killed in the sterilizer.

First I would check that the sterilizer is clean inside. If it isn't,
it can't do its job.

At 25W, you can use the sterilizer for a tank up to around 200gal, depending
on what organisms you want to kill. But, for algae, you will still get more
than
enough zap dosage at that wattage. So, unless your tank is larger than 200gal,
this is unlikely to be the problem.

The flow rate of the sterilizer must not exceed a threshold beyond which the
zap
dosage drops too low to be effective. For a 25W sterilizer, you must keep the
flow rate below 200gal per hour to get a zap dosage of at least 20,000
microwatts per second per square centimeter.

The flow rate through the sterilizer must be high enough to turn over the tank
volume such that most of the tank water passes through the sterilizer at least
once a day. Because the sterilized water mixes with unsterilized water, it
turns out that to get that turnover rate, you need a higher flow rate than
you would intuitively expect. Recommended turnover rate is between once
and twice per day (meaning that 99% of the tank water has actually passed
through the sterilizer at least once or twice a day). For a 100gal tank, the
flow rate must be no less than 40gal per hour, for a 200gal tank, the flow
rate must be no less than 80gal per hour.

So, here are your parameters:

- Check that the sterilizer is actually doing its job (clean, and working
tube/bulb).

- Don't run the sterilizer at more than 200 gallons per hour.

- From your tank size, work out the required turnover rate. For 100gal, that's
at least 40gal per hour (but 80gal per hour would be better), for a 200gal
tank, that's at least 80gal per hour (but 160 gal per hour would be better).

If you stay within those parameters and keep the sterlizer running 24x7,
you will not have green water, period.

I eventually want to purchase my own diatom and UV, but wanted to hear if
others use the diatom 24x7..

No, you can't run diatoms 24x7. The filter media get clogged very quickly,
typically within a few hours. Diatom filters are used as occasional water
polishing filters, not as permanent filters.

Cheers,

Michi.

--
Michi Henning Ph: +61 4 1118-2700
ZeroC, Inc. http://www.zeroc.com