I have a small tank in my yard, 100 gallons, with a few goldfish in it.  The goldfish have been living placidly in this little tank for over a year, and it is filtered by aquaponic grow beds.  The filtering is sufficient enough for my toddler to dump an entire large can of fish flakes in there with no impact.  It clears up in 24 hours from THAT, but I let three baby ducks hang out and splash in this water one day while I was cleaning out and moving their enclosure.  In ONE DAY, three ducklings fouled up the water enough to kill the goldfish.  They were gasping for air at the surface on day two so I changed out half the water and added fresh, but even the excellent filtration of the aquaponic beds couldn't save them.  Dead in a week.