Not realy a difficult algae to batlle, but difficult to determine the cause.. Find the thread "What causes BBA". And you'll find what i mean.
Experiences vary hugely and it makes tons of opinions about it's cause. I would say, read it and pick one you like the most or adds up best to your sutiation. Bottom line, it's true cause is a mysterious guess.
Imho and experiences the main truth in it is, BBA as algae is a plant, it needs light and it needs nutrients, if both add up in it's favor it can flourish. It can flourish in high to low light intensity and anything in between. You say it grows on your substrate, in general this is the place in the aqaurium with the lowest light penetration. It likely still grows there because it's acces to its nutrients is right there and there is still enough light to photosynthesize.
In one of my tanks i experience the same, a relatively low light, low energy tank and it likes to grow bba near the substrate, on the gravel and on pieces of wood and rocks in the gravel. It's the only tank i have that has a sump filter with a low turn over. It only gets skimmed at the surface by the overflows. Ad since it is low turnover it collects a lot of debri in the substrate. It's free floating debri that doesn't get caught by the filter intake and finaly sinks and collects at the tanks bottom. I need to regularly vacuum the substrate of this tank to control the BBA growing on it. The other tanks that have a filter intake submersed with a higher turnover sucking all free floating debri into an external cannister have a much cleaner substrate and never experienced bba growing on it.
Now golldfish are real poopers, they eat a lot and poop a lot. In your case that would be my first suspect.. Do more gravel cleaning, get the dirt out of it, that's most likely the bba's nutrient provider.
For the rest, spot treating with hydrogene peroxide 5% is a very effective method to kill off BBA. This way you can kill of what's in there and than make sure you keep on cleaning out the substrate. Encreasing turnover and lowering light intensity and or maybe duration could also assist.
Best work in stages, don't do all at once.. First start killing off what you have and start cleaning in a regular schedule. If that doesn't do the trick and it keeps growing back, than also lower the light intensity.. And so on and on, till you find that sweet spot where it's under controll.
Good luck..
Ps. The funniest BBA story on the internet is a Youtube video from Rachel O reily. She had a rather large tank with some ancient looking monster fish and a huge carpet of BBA that fitted the setup gorgiously. But one day actualy i do not know how long it was in there. All BBA suddenly without any obvious reason started to die off and within a week she had no single thread of BBA left in that tank. Finaly somebody loving BBA that wants to grow it and can't and wondered why everthing in her tank died without here doing something about it and how to get it back. I believe she never succeded in growing it back.
