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aquatic compost


This amounts to thinking out loud, so don't take it too seriously.

We add fish food and fertilizers to our tanks to promote the growth of
plants.  Of course, the growing plants have to be trimmed and removed from
the tank.  When we trim and discard plants and plant cuttings we remove
nutrients from our tanks that we eventually have to replace with more
fertilizers and fish food.  The fertilizers we use aren't necessarily
balanced to our plants needs, and that can lead to the gradual
accumulation of the unneeded nutrient, some of which are damaging or toxic
at high enough concentrations.  The recent thread on environmentally
aggressive non-native aquatic plants contained the tidbit that careless
disposal of plants and cuttings could also result in releasing those
plants into your surroundings. 

Is there some way we could return the nutrients from our cuttings and
removed plants to our tanks - doing for our tanks what composting does for
our gardens?  This could reduce or eliminate the need for some
fertilizers.  It  might also be a useful amendment for the substrate, adding
stabilized organics and improving nutrient availability in the long 
term.  It's also very "natural" and seems like and essential part of 
maintaining a model aquatic ecosystem, which some of us would like to do.

But how to do it?

The first thing I'd do is cut the the plants and trimming to fit in a 
blender, add the minimum amount of water necessary, and puree.  After 
removing tough fibers, what's left would be a nice green soup.  Nutrients 
in the soup would probably not be immediately available to plants.

So what to do with the soup?

The direct approach would be to just dump it into the tank.  Most would
settle and form a layer on the substrate.  The rest would get filtered. 
Snails and other detritivores would probably think this was manna.  Some
herbivores might like it too.  The direct approach would be risky because
it returns all of the nutrients (including nitrogen that probably isn't
needed in a tank with fish) and they will probably pass through unstable
forms that might cause problems in the tank.  Rapid decomposition of the
organics is probably also going to be a problem. 

You could also put the soup into a media chamber in a filter.  That would
keep the detritivores and herbivores out of the picture and allow slower
breakdown, but you're still likely to get a big slug of nitrogen that you
don't want.  The filter could plug, creating a mess and any substrate 
amendment you would get from the direct approach would be lost.

So how about taking the soup through a decomposition step outside the 
tank - the aquatic equivalent of compost pile?

If you have some spare equipment then you can probably just dump the soup 
into a bucket with a lot of aeration and circulation.  That will allow 
for rapid breakdown and stabilization of the released nutrients.  After a 
few days you'll have a brown soup instead of a green soup and  most of the 
very decomposable organics will be gone.  Some of the mineral nutrients 
will be in the water, and some will remain in the partly decomposed plant 
material.  From here there's lots of options - dump the brown soup in the 
tank, strain the solids and return the liquid to the tank (solids could 
be discarded or returned to the bucket) and so on...

A third option (the one I'm leaning toward) would be to put the soup into
a bucket with mild circulation so that the soup will support
denitrification but not the smellier anoxic processes like sulfur
reduction.  This would reduce the nitrogen content of the soup and might
keep metals in a more readily plant-available form (either chelated or
reduced) than you would get from the aerobic mix.  I think you could 
filter the solids and return a useful trace element mix to the tank.  The 
solids would be kept in the bucket until they reach a nice, stabilized 
form and then used for a substrate amendment.

Any other ideas?