Hi - my first post.
Bonsai beginner - score over about 4 years = 3 (alive)/2 (dead)
Location: London, UK

Need help with the beautiful maple I got for father's day a few months ago please.

I initially kept this inside on a bright window sill that doesn't (much) get direct sunlight.
Our tap water is very hard. The leaves deteriorated about a month ago steadily and has got worse.
Since then, has been watered with collected rain water for the last month and kept outside after reading posts here in a sheltered area (to avoid the wind as much as possible).

The leaves I think have either some sort of leaf spot or fungal infection? Photos attached.

They also exhibited signs of white powdery mildew which I treated with a weak vinegar/water solution also after reading some tips here but the leaves still don't look good.

It's coming into autumn so the colour is perhaps normal for this time of year but the condition of the leaves I don't this do.

Greatly appreciate any tips! My wife bought me this from a nursery and I don't want my 3/2 score getting any worse!

Have read about organic fungicides but after reading their reviews don't want to spray my poor tree and make it any worse.

Thanks for reading!

Look like it has gone into autumn mode a bit early caused by the stress of being kept inside. Nothing you can do about it just leave it and water when needed and next year it will produce new leaves.