Help Me Understand Phosphates! Frustrated

[stanleo]

So when I test my FW tanks for phosphates with the API Master Phosphate kit I get a higher reading right before the water change and a lower reading after the water change. That is what I expect to happen so I am confident that the test kit is working properly. Those tanks are planted (one of them heavily) and well stocked. No algae problems.
 
The issue is the 55gl SW tank. The test kit is for both FW and SW. When I test the SW tank before or after a water change I get a very low reading. About  0.25 ppm. There are only 3 small fish in there and about 20 hermits, a few snails and an emerald crab. I have an algae problem in that tank. Nitrates test at 20 ppm
 
These are my questions:
 
Why do I have an algae problem if my phosphate is that low?
 
What should the phosphates be testing at? In FW and SW?
 
Would buying a Phosphate reactor for the SW tank help with the algae?
 
Is there anything else you can tell me about phosphates?
 
Thanks guys, any help would be appreciated.
 
 

 

[SLIM]

I cant help you with salt water but I can tell you phosphate test kits are so inaccurate that they really aint worth using.
They are even worse than nitrate test kits. I assume your getting spot algae? If so this is a very clear indication of low phosphates regardless what the test kit is telling you. I used to use phophate and nitrate test kits when I first got into aquascaping but soon threw em out due to there inaccuracies. Best thing you can do is just observe your tank for algae and deficiencies and act accordingly.

Also seems you asked the ideal phosphate level should be 3ppm

 

[CezzaXV]

I was told that ideal phosphate levels for SW are less than 0.03ppm. Most folk recommend the D-D phosphate test kit, as it is much more sensitive than others on the market.

Running a phosphate reactor would of course help get your phosphate down, and might help with your algae problem. It might not, because there are other causes of algae, like nitrates and silicates (although if you run a reactor containing Rowaphos, it will deal with both phosphates and silicates). Your nitrates are also a little higher than I personally would be comfortable with. They're acceptable for a fish only tank, but they won't be helping your algae problem.

I do know that higher phosphate levels are accceptable for FW than SW. I don't know why this is, but my guess would be because in a FW tank you have plants and stuff to use up the phosphates naturally, but in SW there isn't really much that will use up phosphate other than algae, which has nothing to compete with once it gets established which is why it reproduces so rapidly.

 

[Donya]

Other than the test kit accuracy issue, when you can't measure a nutrient and have an obvious thing using up that nutrient, that in itself basically tells you where it's going and that you would have move of it if that nutrient sink didn't exist. Algae can easily take phosphate and nitrate down to undetectable levels, hence the use of macroalgae refugiums and algae scrubbers in marine aquariums. If all of the algae were suddenly to disappear from your tank right now, you'd get higher phosphate and nitrate readings pretty fast.
 

I do know that higher phosphate levels are accceptable for FW than SW. I don't know why this is, but my guess would be because in a FW tank you have plants and stuff to use up the phosphates naturally, but in SW there isn't really much that will use up phosphate other than algae, which has nothing to compete with once it gets established which is why it reproduces so rapidly.

 
Aside from the algal bloom encouragement, many corals respond badly to elevated phosphates.

 

[stanleo]

That does help a lot guys thank you?
 
I will be getting that reactor as soon as funds become available. I feel like that is something essential that is missing.
 
But you brought up another question from me. What are silicates and how do you test for them? Should I be concerned?
 
Thanks again

 

[Donya]

In general there's no need to worry about silicates. New tanks experience diatom blooms that consume silicates, but the blooms are brief and managed by snails. Occasionally a more mature tank will experience a small diatom bloom or two but they are nothing to worry about.