Dissolved Organic Compounds

[RickyV]

I am often asked why I do big and frequent water changes in a planted tank if the plants are consuming the excess nutrients. From what I have read on several forum posts is that there is a correlation between water changes and reduced algae and increased plant health. The most common reason said to explain this correlation is that water changes remove dissolved organic compounds.

This is my interpretation of the forum posts I have read so please correct me if I am wrong. Dissolved organic compounds include things like proteins, lipids, carbohydrates that algae can easily utilize as nutrient sources. Plants cannot utilize these dissolved organic compounds as easily (or at all?), and need them to be converted to inorganic nutrients like nitrates and phosphates to use them. Algae does not need to wait for them to be decomposed into these inorganic nutrients so they thrive with high levels of dissolved organic compounds.

I have also read that dissolved organic compounds are removed because they significantly increase the levels of aerobic bacteria which can reduce the oxygen levels in an aquarium. However is this really a concern with enough gas exchange in the tank? Or is it possible for the bacteria population to get so high that it would take a very unreasonable amount of gas exchange to supply enough oxygen to keep DO at optimal levels?

This gets me to my next question, if bacteria are consuming DOCs then they are competing with algae, which would be good? So would another method to assist in the removal of DOCs and thus reduction in algae be to use very large wet dry filters? Then plants utilize what is generated from the wet dry filter. I actually don't use any bio media in my planted tanks, however this is making me rethink that.

 

[Bradders]

Really keen to read the responses to this question! It is a great question.

Especially that you don't use biomedia in your aquarium environment - begging the question where all autotrophic and heterotrophic critters are colonizing.

 

[LMuhlen]

What I imagine on this topic, with no real evidence to back it up, is that the DOC trigger algae, but not necessarily is consumed by algae. I picture the DOC being processed by microbes and degraded into nutrients that both plant and algae can use, but the algae are "smart" and know that DOC will mean nutrients down the road and get triggered by it.

 

[bazz]

Really keen to read the responses to this question! It is a great question.

Especially that you don't use biomedia in your aquarium environment - begging the question where all autotrophic and heterotrophic critters are colonizing.

All of the surfaces within your aquarium is biomedia, I know biomedia is the given name for some porous bits of stone or round plastic balls but you are basically just providing a surface for bacteria to colonise, and it's then a case of providing the right conditions for the beneficial bacteria to proliferate. The magic of a planted tank is the plants themselves are already taking up some/most of the waste produced/administered but they aslo oxygenate the substrate creating a massive filter bed. The plants microbes live a symbiotic relationship and feed each other what they want. Then you have the surfaces of the leaves and hardscape even the glass if you haven't just cleaned it. These processes don't happen so freely in a well stocked fish only system this is why you would be required to use large amounts of filtration/biofiltration. Admittedly I'm using a 2180 on a 90x55x53 but I've had it for about 10 years now, there is only a single layer of SubstratPro in each of the three baskets and I'm mainly just using for flow. When/if it gives up the ghost I will replace it with a much smaller filter and turn the Nero 3 up a notch two.