Hi everyone

You're very welcome here Wylam.
For even a small pond you will have a much more stable environment water-wise than a tank so you needn't be doing such drastic water changes.
Those are much more important in a more confined area of an indoor tank.
Your pond is a very small one but being outdoors (presumably) the temperature will be less of an issue (although the Summer seems to have finally arrived here in the Midlands).
I really don't think you should add to your fish stocks for the time being - if at all, Goldfish (including shubunkins) are pretty fast growers and emit a lot of waste to boot.
I do suggest you get a small outdoor pond pump and filter though as this would be the secret to getting the pond water properly cycled...without an effective filtration system cycling cannot properly happen (although plants can absorb a lot of nitrates - but there's little hope of the ammonia produced from the fish's waste actually getting turned into nitrates).
Many pond plants these days are sold in small 'open' mesh-type pots and can be submerged into the water 'as-is' I really don't think you need to be worrying about adding any further substrate than the gravel you already have, plant roots will grow through the mesh and find their way into the gravel, rest assured.
Until you do get some form of filtration running be as frugal as you can with the fish food - once a day is sufficient - don't feed them more than you see them consume (although they will 'eat and eat'!!!) so limit it to what they eat in a few minutes.
Sorry, I just re-read your post and see you do already have a filter.
Do you have water test kit?
Really you only need individual one - Ammonia, nitrite and nitrate...you could go for the pH as well, but Goldfish are pretty tolerant creatures and will live in pretty much all water - but preferring slightly higher pH.
I hope this helps - any more questions...fire away, always someone to give advice here.
John

:o)
Nemo, that's really great.

That's an excellent idea to have a bit of a 'retreat' for all four.
The readings for the ammonia should always be as near to 'nil' as possible - better to be zero really.
Likewise for nitrites.
pH for Goldfish is better on the higher side of neutral (pH7) and since your water is hard and 'limey' this will be 'just the job'...but if you plan on topping-up with rainwater you might need to keep an eye on this over time.

Frogs - you either love 'em or loathe them!
The ones which visit my pond have never brought anything untoward with them (other than themselves) so you can leave them to come and go, I think.
What might be a problem would be if any 'pass on' and their corpses remain in the water...just keep and eye open for such things - likewise land snails and slugs which fall in and mightn't be able to escape and drown. These, too, can pollute the water.
Back to frogs - I have read of males (in the absence of females) 'latching onto' pond fish (amorous little lads that they are) and causing problems - but I think this was more towards 'fancy' goldfish and I have never personally witnessed this.

John

The water in my pond didn't freeze all that deeply last winter or the winter before.
I did keep the pump running all winter though (something else the experts say shouldn't be done - I think it didn't freeze much more than four inches.
A half-metre depth isn't ideal, but I would chance leaving them out - especially if you have made a cover.
Whatever you do do - dont break the ice! It causes terrible vibrations in the body of water under it and can damage the fish.
The best thing to do would be to put a large saucepan of boiling water onto the ice and eventually (you might need to 'refresh' the water in it) it will melt a hole through it.
This will allow the gases which have built up underneath it to escape.
Some people use a ball which sits in the water surface and the ice supposedly forms all around it...I think the ice here hasn't read that advice as the ice formed and pushed the ball up until it sat in top of the ice!
You can buy a pond heater which might be worth considering in a small pond such as yours and mightn't be too prohibitive to run...but for now just enjoy the fish through the nicer weather.
Oh yes - you don't feed outdoor fish when the water gets cold - below 50f is reckoned to be the cut off point. They will still eat the food at lower water temperatures, but seemingly it passes right through them undigested (they don't seem to be able to digest food below this temperature) and can then pollute the water.

John