Don't get any more Fish!
Your pond is only going to be just big enough to suit the fish you already have.
An 8'x4' pond is 32 sq feet of surface area and the guidance is for an inch of fish for every sq foot of surface, so as you already have one 14" fish, that leaves only 18" of 'spare capacity' which is probably about right for the other 4 fish you have that are 4-6".
Also I can see loads of koi sticks floating around the edge, the water is likely way too cold to feed them yet, if they do take the food it is likely they wont be able to digest it and it can do them real harm as at low temperatures it is just rotting in their gut.
Water temperature needs to have been above 10C/50F for 2 weeks before putting food in (and just a quarter of the sticks I can see) and never let any stay in the water more than 20 minutes to pollute it - it'll be rancid enough in that little pond with so many fish and only a waterfall for circulation and aeration, any filter you might have under those stones can't possibly be anywhere near big enough for that stocking level.
To do it properly you really need a filter about 10% of the pond size so 16cu ft (ish) of filtration, a pump that will shift about 1/2-3/4 of the water in an hour (ish), also a UV clarifier (not instead of filters) probably a cyclone unit before the filters to separate out all the crud (or you'll be cleaning foam forever).
A ready built unit the Cloverleaf CL1V is a good filter combo and very compact at about 2 foot cube so actually 50% undersized but 'punches above it's weight' and a snip at ?350 (plus media) and might just be good enough - obviously you still need the pump and clarifier and pipe (and safe power out there).
Spend at least ?70-100 on UV (15W+) and a pump (depends on head of water, if pump on floor then 5' head to waterline - if filter in ground then 7' etc but filter needs about 500-800gallons an hour but that's at whatever the head will be (say about ?200).
Do not buy any filter with hozelock or lotus written on it as they are only toys.
Oh and the pond will cost ?100 a year in electricity too.
Strengthen the corner plates as you'll have over 4 ton of water in there...
Your pond is only going to be just big enough to suit the fish you already have.
An 8'x4' pond is 32 sq feet of surface area and the guidance is for an inch of fish for every sq foot of surface, so as you already have one 14" fish, that leaves only 18" of 'spare capacity' which is probably about right for the other 4 fish you have that are 4-6".
Also I can see loads of koi sticks floating around the edge, the water is likely way too cold to feed them yet, if they do take the food it is likely they wont be able to digest it and it can do them real harm as at low temperatures it is just rotting in their gut.
Water temperature needs to have been above 10C/50F for 2 weeks before putting food in (and just a quarter of the sticks I can see) and never let any stay in the water more than 20 minutes to pollute it - it'll be rancid enough in that little pond with so many fish and only a waterfall for circulation and aeration, any filter you might have under those stones can't possibly be anywhere near big enough for that stocking level.
To do it properly you really need a filter about 10% of the pond size so 16cu ft (ish) of filtration, a pump that will shift about 1/2-3/4 of the water in an hour (ish), also a UV clarifier (not instead of filters) probably a cyclone unit before the filters to separate out all the crud (or you'll be cleaning foam forever).
A ready built unit the Cloverleaf CL1V is a good filter combo and very compact at about 2 foot cube so actually 50% undersized but 'punches above it's weight' and a snip at ?350 (plus media) and might just be good enough - obviously you still need the pump and clarifier and pipe (and safe power out there).
Spend at least ?70-100 on UV (15W+) and a pump (depends on head of water, if pump on floor then 5' head to waterline - if filter in ground then 7' etc but filter needs about 500-800gallons an hour but that's at whatever the head will be (say about ?200).
Do not buy any filter with hozelock or lotus written on it as they are only toys.
Oh and the pond will cost ?100 a year in electricity too.
Strengthen the corner plates as you'll have over 4 ton of water in there...
Sir Alex Ferguson. 
(20)
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