Phosphate & Nitrate Levels And Algae

Hello All,

My 125L community tank is now 9 months old. Initially, during my very protracted fishless cycling, I ended up with a lot of green and brown algae, and nitrate readings "off the scale". After cycling, I quickly added otos and panda cories. Both thrived, and the algae disappeared  . About 3 months ago, I noticed that it was reappearing, so I added 6 Amano shrimps, but the algae has continued to thrive. At that time my nitrate levels were about 40 ppm. So, with only 5 ppm in my tap water, I decided that I must be overfeeding, and I reduced the feed by about 50%. Nitrate is now 20 ppm, but the algae situation has not improved.  . Also, the "pan scourer" glass cleaner which I got from MA is ineffective. It just rides over the surface of the green spots on the glass.

Today, a very battered API phosphate test kit arrived from Amazon, and I tested both tap and tank. To my amazement there is more phosphate in the tap (2 - 5ppm) than there is in the tank (1 ppm)! I know that at this time of the year, after farmers etc. have fertilised their fields both phosphate and nitrate can and do end up in our tap water. Also, water authorities add phosphate to "line" the inside of any lead pipes still out there, to reduce lead poisoning.

Otherwise, my heavily planted tank is thriving, and the otos etc. are just as busy as ever. I have read much about algae on here and elsewhere, and as usual, some of it is totally contradictory. Any thoughts as to where I go from here?  MY current feelings are to sit tight for another 2 months, and see if the algae eventually reduces. I welcome any thoughts and suggestions.

                                                  Skittler

My research indicates Algae is caused by an imbalance between the three things needed for plant growth:

• Nutrients (includes Phosphate, Nitrate etc)
• light
• Carbon (this can be CO2 injected or dissolved liquid carbon)

It's hard to say what the cause is, in each individual case, as it could be too much or too little of either Nutrients, light or Carbon.

Do you add CO2 or liquid Carbon?

How long are your lights on for?

Do you add plant fertilisers - what dosage?


The fact your aquarium has lower phosphates that your tap water indicates that your plants (and algae too perhaps) is consuming some of the phosphate from the water column.

I had a bloom of Brown Diatoms and added some Phospate Remover (the product I used was called Rowaphos) and that seems to have eliminated the problem for me, however I also increased the fertiliser dosage at around about the same time too.

Liquid carbon is one of those things that a lot of people refuse to have anywhere near their tanks. Most of them contain a form of glutaraldehyde which is toxic - that's why vallisneria doesn't like it. Neither do some inverts and fish.

Don't forget that all tanks have some algae, it's when it gets out of control that something needs to be done. I  scrape the front of my 180 litre, which has otos and nerites, every month. The betta's tank needs doing too, that has one small nerite. But the 50 litre litre doesn't need it - shrimps and a nerite. This could be due to the different amount of light on the tanks, the numbers of fish (and therefore different amounts of fish food) in each tank, and the different levels of planting.
I'm not well up in plants but if yours are thriving you are doing something right. If they are easy plants, are they also low light plants? You could try dropping to 6 hours and see if that makes a difference.

I don't believe Glutaraldehyde is toxic at the recommended dosage.

Most things are fine with the recommended dose but some plants are sensitive to it.

Quote from here

The ones I'm aware of are Egeria Densa, Riccia, Vallis and Fissidens. Some people also report it affecting shrimp and otos

I was advised on another forum to use liquid carbon, and it knocked 7 bells out of my Vallisneria - which I have since read is to be expected. That's why I joined Thinkfish ....... My filter really moves the surface a lot, so I doubt that there is a shortage of CO2.

Vallis are noted for their intolerance of liquid carbon.

Surface agitation decreases CO2 levels. I don't know why it has the opposite effect to oxygen, it's some chemistry thing and mu science background skipped chemistry I'm afraid.