We are in zone 5 and have a 4 year old pond, 53' x 55' and about 4-5'
deep (about 100,000 gallon pond). We put in a 5000gph pond pump just
to circulate the water and that works great. Yours wont need a pump as
big but should have something circulating the water in some way. We
fountain ours (until the waterfall & bog is complete later this year)
to break the surface water. There will be no issues with stagnation or
mosquitoes with the water moving. As far as the filter issue goes from
experience I can say that plants are the most important factor,
espically if it is in full sun. By late summer your pond will be pea
green if in the sun (but still 100% fine and alive) from algae,
espically if new. Many, many live plants in a pond will do much more
than a filter. Trouble is it takes a lot of time to 'grow' live
plants in a large pond unless you can spend a ton of money on plants
and have them survive. Pond lilies are the best because they block the
sun and use nutrients that the algae thrives on. Our pond still is out
of balance, looking at doubling our pond lilly population this year and
the other plants are just now starting to take over the pond. It has
been trial and error to see what plants will survive in our pond. The
rain mostly fills our pond but in dry spells I drag out the garden
hose. If you have questions email me and I will gladly answer from
experience.