WD wrote:

First may I ask, Why no filter? Some kind of filtration is needed in almost
all tanks, if only to create a bilological filter bed to convert the ammonia
to nitrogen.

My daughter's small betta tank has no filter. 50% water change weekly, and the
java moss eats lots of nitrate and grows fast.

My 72 gallon show tank has a high fish load and no external biofilter. The
cannister mechanical filter is cleaned too frequently to allow a biofiltration
colony to get established. The tank is very heavily planted, so the plants eat
most ammonia. What ammonia they miss the bacteria in the gravel get-it hasn't
been vacuumed in 14 months.

However, my quarrantine/breeding tank has a sponge filter with a
long-established biofiltration colony. It gets squeezed in waste tank water just
once every few months. Not enough plants or gravel to make a difference in this
tank, so a carefully established biofilter is necessary. Just a sponge
though-fry can't handle power filters.

As has been said often on this list, plants make the best biofilters.