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Discussion Starter · #1 ·
Like a lot of people, my tap water contains phosphates. From what I understand, it's the result of the water company and the various things they add, or the breaking down of various things they add.

I'm no chemistry expert.. far from it, but I recall reading something a while ago regarding phosphates. Because of what I *think* I know, I've made assumptions. I would like to get a "fact check" so to speak...

Macro fertilizing is NPK... Nitrogen, phosphorus, and potassium (feel free to correct me if I'm wrong. That's why I'm posting it)

In moderately planted aquarium, I ran two tests. Nitrate and Phosphate. Neither test has been calibrated to ensure accuracy, however, I did test my RO water (not used in the aquarium) to ensure that 0 was 0. It was.

The nitrate test showed around 10 ppm. The tanks is pretty well stocked. The test was done at the end of the week before their 50% water change. The amount of nitrate the test showed wasn't as important as the fact it showed a presence of nitrates.

This tells me my plants have more than enough nitrogen for the time being. This number has been going down and I'm going to keep watching for signs of nitrogen deficiency as a couple of new plants grow. (Again, correct me if I'm wrong)

The phosphate test showed a smidge under 0.5 ppm phosphates. Based on the nitrate test, I at first though "Ok, plants have enough phosphorus" Then I recalled something I read about an algae filter in a salt water aquarium. Someone said something to the effect of: There are two kinds of phosphates. Organic and Inorganic. The protein skimmer removes one, the algae removes the other. That got me thinking...

I tested the tap water, it tested at an even 0.5 ppm. Again, the actual number is meaningless since the test kit isn't calibrated, but it was slightly larger than the test of the aquarium water. I was also lucky enough to be cycling another aquarium with fish food (no fish, just dumping in food so it rots and produces ammonia) Assuming fish food introduces phosphates, I tested that and it was between 5.0 and 10.0 ppm. Assumption confirmed since fish food was the only thing being added to the aquarium.

My conclusion from the series of tests is as follows: My tap water's phosphate content is primarily composed of phosphates that are unusable to plants, the phosphates being introduced from feeding the fish are all being consumed by the plants, I should start dosing phosphorus are the normal part of my NPK dosing.

I know, I could of just posted my conclusion and not had a bit long winded post about rudimentary science, but I like to show my logic behind arriving at such conclusion. So, the real question is, am I right or wrong?

Thanks for reading :)

· Registered
3,350 Posts
I like your analysis. A lot, actually. :)

The consensus answer in this hobby, or at least in this forum, would be:

1) No distinction is ever made between nutrient forms. All phosphates are treated the same, and there is no reason to distinguish between them.
2) At least 20ppm nitrate, 2ppm phosphate, and 20ppm potassium are required to avoid deficiency in all plants. As levels drop below that, progressively more plants encounter difficulty uptaking nutrients at sufficient rates.
3) However, tests are too inaccurate to be of any use in determining whether you actually have these minimum levels. Even if they were accurate enough, it's a waste of time to test. Instead, simply add an excess of all nutrients, such that there is no possibility of any nutrient dropping below the minimum level.
4) The excess nutrients do not cause algae.
5) Although eventually, nutrients will build to toxic levels, and large excesses (as well as other accumulated wastes) are periodically removed by performing a large regular water change, typically 50% weekly.

As far as advice goes, the above is practical and works almost universally. I use it in my own aquariums, and you should too.

However, if you want to experiment, look a little deeper, and try to draw a few extra logical conclusions, you will find the success of the same advice will hinder you, because it is accepted as gospel. The common responses to anything that challenges it as incorrect or incomplete are dismissive (why bother when what we have works), nonsensical, and sometimes even hostile.

Here's two replies I see a lot. Given your analysis, I'm sure you can spot the logical errors within:

"When plants are healthy and growing fast, they out-compete algae for nutrients." This comes from people who dose nutrients in excess, such that none are ever limited.

"I've dosed huge excesses of potassium phosphate, far beyond anything an aquarist would encounter. No algae resulted. Therefore, excess phosphorus does not and will never cause algae." This one comes from the same aquatic biologist widely credited with developing the advice I listed above.

Since you're new here, I figured you should know what you're up against in attempting to start a discussion of this nature. :) I typically avoid it myself, but like I said, I liked your analysis.

That said, I agree with you at least in theory that nutrient form matters. I have repeatedly witnessed different forms of phosphorus having particular effects not accounted for by present theory.

However, I don't know if the phosphate form in your water supply is in fact unusable by plants. The person here most qualified to give you a scientific answer is the same biologist I mentioned. I have attempted to open discussions regarding different phosphate forms with him and others in the past, and received only answers that don't address the question, or contain further logical errors that make the response suspect at best.

I do think that you cannot rule out the possibility that in your aquarium, phosphate becomes so sparse at 0.5ppm that absorption might simply slow to the point where it effectively goes no lower.

· Registered
21,007 Posts
Two ways you can "feed" your plants:
1. Determine what they need and try to dose just that amount of each nutrient.

2. Try to maintain more than they need of every nutrient at all times, so the plants are never limited in how they grow by any nutrients. This is most difficult with carbon.

Both methods can be made to work. It is up to each of us to decide which one we wish to use.
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