i do hope your still paying attention to this thread.
i'm not so much looking at a shrimp tank to be self-sustaining, but one that could include fish.
our nitrogen cycle (the overly simplified ammonia, nitrite, nitrate, nitrogen) bacteria can reduce it to nitrates, plants can use the nitrates. everything i've come across plants want nitrates to be about 40 ppm
in the wild, aquatic wild, everything starts with phytoplankton. (if you ignore bacteria have to move the nutrients into the water first - but we have the bacteria from our nitrogen cycle to help us out there)
phytoplankton feed zooplankton, zooplankton feed our fish.
there are hundreds of zooplankton in the world that are suitable for all shapes and sizes of fish. today, the market shows 4.
-rotifers - unlabled species
-moina - unlabled species
-daphnia - pulex
-daphnia - magna (expected default your going to find)
-copepods (we're rather large by this point, far to big for me to consider use for a self-sustaining tank - so i'm not counting it in the list)
rotifers are tiny, but little inherent nutritional value, only from the phytoplankton they eat.
after rotifers moina exist, but these may not be available to you due to the country you reside in.
daphnia, either species, pulex are over 3mm. manga are 5mm for adults.
you need a fish that looks at these species with a mouth big enough to eat them.
moina and daphnia have significant inherent protein content as food
moina are about 1/4 the size of daphnia, magna species
one lady i spoke with had a single 4" fish with a carnivore tendency that ate all the daphnia in her tank within 3 mo. (30 gallon)
so you've got an idea how much a single 4" fish will eat.
you've got an idea of how large a fish is to eat daphnia.
an idea of tank size requirements.
bacteria doesn't complete the nitrate cycle in our freshwater tanks. but plants can directly take in nitrates. plants also process CO2 and add O2.
plants (i'm hoping) will provide sufficient hiding for zoo plankton.
the smallest omnivore with herbivore tendency i have heard of is the flag fish (Jordanella floridae)
strictly herbivore fish are drastically larger, at 4" being the smallest i have ever seen. herbivores also consume a considerable quantity of plant matter (2 reasons to have a larger tank)
and then there are detrivores, (blackworms & snails)
i'm hoping plant eaters will give the zooplankon a better chance, i'm hoping detrivores the fish can eat will take the load off the zoo plankton. ...
with 2 flagfish, i'm hoping a 40 gallon breeder will be large enough. if not, something is going to give and the whole thing will fall apart and everything will die.
and aim for a PH of 7.5, this won't hide all of the ammonia in an ammonium form, but should be high enough, so any sudden changes there isn't enough to ammonium to kill your tank by spontaniously turning into ammonia, ... a PH of 6.5 ... a sudden change to 6.7 your going to have dead fish all over if you have enough ammonium present, ... and our test kits don't test for ammonium. i lost half my fish this way before i figured out what was going on.
ghost or cherry shrimp for a self-sustaining idea is another way to go, ... make sure you have sufficient algae growing and cut out everything else in the above equation. this could be done in a much smaller tank.