# Effects of light on anoxic substrate conditions



## ChuckM (Mar 19, 2020)

I'm wondering if in order to create anoxic conditions in a deep substrate is it necessary to exclude all light.

The situation is a 6 gallon Fluval Edge tank with about 1-1/2" baked clay substrate over a 3/4" plenum (repurposed undergravel filter grid). I have a larger tank with a 4" baked clay substrate covering a 1" plenum and nitrate levels stay under 40. Both builds are just over 2 years old, medium planted and stocked. But this Edge tank shows nitrate levels up to 100 and phosphate around 10, that don't seem to come down on their own. I'm not convinced it's worth chasing nitrate levels unless they get really out of control, but this would tell me something's possibly not working right in my anoxic experiment (it's also possible I'm just overfeeding).

The light part of my question is due to the design of the Edge. The tank sits on a kind of pedestal that leaves about 50% of the bottom exposed to ambient (albeit shaded) light. I'm thinking of maybe applying a layer of black-out attached to the underside and up the sides about an inch or so. I can't find anything on the webs about the effect of light on facultative anaerobes.


----------



## mistergreen (Mar 3, 2007)

Your tank might have little organic c or co2 which they use for energy.


----------



## ChuckM (Mar 19, 2020)

mistergreen said:


> Your tank might have little organic c or co2 which they use for energy.


I am putting CO2 in via a Fluval device that takes CO2 cartridges that outputs through a Jardli diffuser directly under the HOB outflow, but my drop checker never turns green (and I'm familiar with how unreliable those are).

Plants include 5 swords, some hornwort and duckweed which all appear to be healthy.
Livestock: 3 Corys, 2 Siamese (genuine) algae eaters, 2 Dwarf Raspora, 1 Bristle Nose Pleco, 2 Nerite snails, 4 Amano Shrimp, 2 Otos.

edit to add: I have heard of adding a bit of Vodka or similar as a carbon source, but alas there's no Vodka in the house. No one here cares for the stuff.


----------



## mistergreen (Mar 3, 2007)

Your tank sounds overstocked lol. That's probably the source of the high NO3. The deep substrate & plants are doing what they can but not enough.


----------



## ChuckM (Mar 19, 2020)

mistergreen said:


> Your tank sounds overstocked lol. That's probably the source of the high NO3. The deep substrate & plants are doing what they can but not enough.


Yeah, I'm open to that possibility too. The Algae Eaters were never meant to be permanent. They served their purpose to knock down some BBA and may be time to go with their siblings in the big tank now.


----------

