As the nights start to draw in, you need to be thinking about using artificial lighting as you want to extend the day so that your young Goldfish in particular can continue to eat for long periods and therefore continue to develop, grow and prepare for winter.
Ideally you will start to time the lights to come on just a little before the natural daylight reaches the fish and you will bring that time forward each week as the days shorten. Similarly in the evening, you want to the artificial lights on before the natural light ends so the fish are not left in the dark and then suddenly faced with lights coming on. The Goldfish will get used to the timings and you should aim to maintain the overall day length at say 14 hours.
It doesn’t matter if the lights are not especially bright, nor does it matter if they are or are not intended for aquarium use. What matters is that the Goldfish can see their food and don’t settle into resting mode too early. The most effective I have found are fluorescent tubes as they provide a wide distribution of light at modest set up and running costs. However, be careful with any lighting that it is waterproof because as the days shorten condensation will become a factor and will water will condense on the light fittings.
Of course, you also need to time the feeding so that it is shortly after the lights go on and ideally well before they go out. The Goldfish will get used to the timings but as the natural daylight decreases so you will need to lengthen the use of artificial light to maintain the overall day length at say 14 hours. If because of your lifestyle, you can’t feed the fish early in the evening then you may need to maintain a longer day so the fish can eat before they are in darkness.
As we progress towards winter the overall day length can be reduced as feeding decreases so that your Goldfish become accustomed to shorter days as well as lower temperatures as they approach hibernation.