Roach without doubt - almost all the waters I fished as a kid were full of them - usually stunted
Lots of nets of silvers more recently - many of which will have been predominantly roach - probably 100+ each time.
Think you are thinking of stickleback. A full grown minnow will flipping attach themselves to a size 14 with a triple maggot no problem! Can’t stand them give me a bleak over them any day and that’s saying something
Two weeks ago I took the grandsons down the Derwent and they caught their first fish (minnows) on two metres of whip, 3lb line and a size 18. My dad took me down there to catch my first fish 52 years ago and I took my son down there to catch his first fish 25 years ago, you could say minnows run in our family.
Minnows aren't fussy. I use often big size 12 hooks and large bunches of maggots in an effort to avoid them. The only thing that happens is that I don't hook quite as many as when using smaller hooks. Sticklebacks are a different kettle of fish. I've had very few over the years. The few I've had have been caught on pinkies and squatts on hook sizes from 22-26.
Has to be roach, when you think about it they must be one of the most common creatures (not including insects) to live in the UK. I was thinking about my local canal which is chock full of gudgeon thrying to work out how many live in a ten mile stretch - thousands of them, can't get through them somedays yet there's more roach there, the lake round the corner - roach, ponds, reservoirs, drains, slow sorry looking urban rivers, big rivers full of boats, natural rivers - roach and loads of them. Tidal stretches too, Even carp puddles have more roach than carp. From the lochs of Scotland to the Southernmost extremities they are found in huge numbers, Also you can get a specimen in any area of the UK and there's not many species that can apply to. Only place they don't seem to show is fast upland rivers and certain trouty beats .
A doff of the old red hat to the mighty redfin they certainly brighten up any catch for me
Back when I used to fish canals week in week out, Roach weren't that common, I mean they'd feature quite high in a list of species I've caught as there's usually a few in most waters, but it was Gudgeon that made up the weights. That's why I say Gudgeon.