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Weekly water change.....how much is right?


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I have a 14g that I run with a 3g refugium, no skimmer. I also use Seachem Matrix in a bag inside the fuge. I have been battling cyan and hairy algae for a few weeks. Been doing 50% weekly water changes to battle this but can't seem to win. So I'm beginning to wonder if I'm doing too much of a water change and stripping the tank of beneficial bacteria. If I do not dose Part A/B or Mg and I just rely on water changes how much water change is right for this size tank? My parameters Alk, Ca, Mg have been stable and tank has primarily zoas, shrooms and LPS. Only 2 small monti frags. Have not tested Ph/NO recently but last time I did 4 weeks ago they were low too and I was still having algae issues. I just got a bunch of red legged hermits to go after the hairy stuff and reduced my LED red intensity for the cyanos. Any other ideas?

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You aren't removing any bacteria by doing 50% water changes. Bacteria lives on the surfaces of everything in the tank, not the water column. Most people do around 10-15% water changes.


How is the tank stocked with fish? How often do you feed and how much? How much flow do you have in the tank? What water do you use, RODI, distilled, tap?

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I use RO/DI water which I also use in my 100g where I have only a mild cyan issue (been running that tank 2+ years). I feed the fish once daily and very little. Use 3 drops of ROE or < 1/8 inch cube frozen. I have a 6-line, yellow coris, naked clown and sand sifter goby. They are all < 1 inch long except for the sand sifter.

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i change 100% over 500 times so the 50 is no prob. there are people with cyano and varying degrees of water change, any of the common methods can battle it independent of your wc percentage.

a water change exports bacteria from the water column and it doesnt matter to the tank.

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I do 10% every two weeks.
BUT, I also used peroxide to eliminate my GHA.

My tank is also 1.5 years old.

There are many, many factors involved and my method

may not be right for anyone else. Trial and error is part of
the hobby. Good luck. W-

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Changing 50% of your water only exports 50% or less of your nutrients (I say "or less" because you may be introducing new nutrients during the water change). If your feeding and bioload keeps nutients up high enough, even removing 50% of them every week may not be enough to keep algae and cyano in check. For a 14g tank, you have a heavy bioload and feeding daily in such a small volume is a lot of nutrients.


You need to measure your phosphates accurately (borrow a Hanna checker) and your nitrates (a salifert or red sea pro test is good) to know what is going on. You may need to run some additional chemical media to keep up with the amount of nutrients you are putting into your tank and without testing, you will never know.


If I put in 0.1ppm of phosphates per week and change 50% of the water every single week, in just 2 months I will have 0.1ppm phosphates AFTER my 50% water change until algae catches up. It'll eventually spiral out of control until you have nothing left but algae. You need to know EXACTLY what you are putting into the tank. Measure your phosphates in parts per billion after your water change and again before your next one - because unless you do something about that, the only thing left to utilize it is algae.

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