Biochar as an aquaculture filter

I'm hoping to use biochar as a disposable filter for an aquaculture tank. The water from the tank would be splashed over and through a bed of charcoal by a pump and fountain, and then flow back to the tank. Every so often the biochar would be removed and buried in the garden, thus putting the accumulated nitrogen to good use. I'm hoping the charcoal will act as both biofilter to convert nitrogen to ammonium and as a filter.

What would be the potential downsides of this? Could the fish be exposed to dioxins? Tiliapia seem to prefer alkaline water; could the water get too alkaline due to the char?

I'm going to set up a small scale test with goldfish in an aquarium soon.

As far as i know bio char is different than whats used in aquariums so i would defiantly test it out on gold fish first . i used charcoal to clean tanks for years and i have to say i doubt you will do any damage if it doesn't kill the goldfish in a few weeks. the way i eventually used it was to help clean any exes nutrient that plants couldn't take up fast enough. i found that plants trumped it every time unless the tank was immature or my system was over fed. i can say it IS a great bio filter when in the tank over a month . by then it is not a good chemical filter though . so i always kept some undisturbed and some room in the filter where i could easily use fresh stuff as a quick fix for water clarity or obvious chemical imbalance that was life threatening to my stock. lastly i think its important to mention charcoals pores clog easily with sediment which can make it almost totally useless in some cases. not knowing that caused me endless grief in the first few years. but the simple solution is to put it in a filter sock then wrap the sock in mechanical filter material thats fairly fine like polyester filter floss and clean the filter floss periodically with untreated water of similar temp to you tank. good luck . im looking forward to seeing what affect biochar has what i used was way more expensive ecologically speaking !

Good point about keeping some undisturbed for biofiltration.

As far as keeping it from getting gummed up; would I be able to change the surface layer frequently to remove the sediment? Or would you still recommend putting it in a filter sock? That might make it easier to change out, anyway.

I'll update the thread as I go.

As far as biochar being different, it may well be. It might have more volatiles  left (though my TLUD seems to do a pretty good job burning them all off) and the pH and other qualities may be more variable.

Yes you can totally use a peace of filter as a pre-filter in the pump. the more you clean it the better as it will keep the chemical filter and the bio filter more efficient. i used a filter sock for it being easier to swap out. charcoal gets everywhere when not contained which can lodge in a pump impeller and cause a failure i had that happen quite a few times. but regardless i think just about any filter could be configured to keep you bio char working longer. the main difference i was talking about was activated carbon is steamed. im not even positive what difference this makes but i do know any charcoal that isn't rinsed before going into a tank will likely cause an algae bloom from phosphorous.i think its important for me to say at this point. all of my warnings come from mistakes i made over a decade keeping fish and my solution to mitigate the issue was almost always plants, a water change , cleaning the mechanical filter or all three.