Plant biomass effects over time on Current and Filtration in planted tanks

Tom Barr

Staff member
Jan 23, 2005
There is an inverse relationship between plant biomass on current and and filtration.

As plant biomass increases=> the current is reduced greatly.
However, with increased biomass, comes increased filtration in the form of PO4/NO3/NH4 removal and increased O2 production.

This is evident as the aquarium now has progressively more and more plant biomass, which progressively has higher light, higher CO2 and nutrient demands.

Light demands are met through growth towards the light source which increases at the inverse square law(very rapid rate of increase).

However, CO2 is rarely added to keep up with this increase in light as the plants grow.

Likewise, nutrients often are only added at some set point and then applied no matter what the plant biomass might be. EI works well because it adds plenty of extra, as does sediment based nutrient sources, there's always a good source no matter what the demand may be.

So the main issue for aquarist really is the effect of plant biomass increase on CO2.

At the start of the new planted aquarium, plant biomass is very low, the filter is generally very poorly filtering the water. If we use a nice seasoned filter from another tank, add mulm etc, do a dry start method prior etc, the filter issue is reduced.

If we add lots of stem plants, then the filter issue is greatly reduced since the plants are much faster at the filtering than say a few little pieces of HC in an open style rock layout. Biomass increases, as it does the increase in filtering abilty of the plants and less reliance on the filter itself.

However, at some point, the current in the tank starts to decline and a great deal, so much plant biomass and little flow=> less CO2 relative to plant demand and less CO2 mixing as well.

You have a lot more plant biomass, it's a lot closer to the light, so you will have a lot more CO2 demand. But if you simply add more CO2, without addressing the current, you get high slug doses of CO2 gas and no mixing= bad conditions for fish.

The solution is not to reduce CO2 and think the algae or poor plant health is related nutrients or color temp etc or tap water...........the solution is to increase current and then see how the CO2 increase helps.

Next, you can try to prune the tank much more consistently at a mid level of biomass. Stay on top of it and see how week the nutrients, CO2 and light stabilize. Pruning has a very strong effect on light, CO2 and nutrients as you can deduce from all this.

An amazing amount.

So what can we do to deal with this change and issue through time?
Several things/ trade offs come to mind:

Prune more often
Increase current
Over biofilter initially at the start up phase of an aquarium
Adjust CO2 as you adjust nutrients (Rarely/never done for all the bantering that goes on over adding just enough fertilizer)
Reduce the light(appears in virtual any and every solution to most issues)
Chose easier slower growing plants
Add more hardscape materials

I think such concepts help put the idea of how an aquarium "balances" and what factors control the balance(both good and bad) through time.


Where current of 1 = 100% full rate, and a current of 0 = no flow
Plant biomass is in dry weight in grams per meter squared.

As plant biomass increases, flow decreases.

Tom Barr