Urgent help required.

Hi guys, I will try not to bore you when it comes to my goldfish Stanley.

For information, over 1 years old in a 75gal tank.

I've had Stanley for over a year now, always a vibrant and bouncy little dude. Eats his food as always on a strict timely and daily basis. Enjoys swimming g through his house and I do a 20% tank clean every 2-3 weeks. And clean the filter the same (not in tap water).

Every time I do so, I do a water strip test and all readings look accurate. Except for hardness which is a bit on the higher end of the safe zone.

Over the last week I noticed Stanley was a bit less active and staying on the gravel bed throughout the day just lying there (not on his side) but as normal, standard bobbing and normal breathing, but its not like him.

I then; upon further inspection noticed that there is a very very slight white film across his gills, almost like a pearlescent paint job.

And when he eats, he spits a lot of it out.

I do not have a picture as I would now have to use flash and I do not want to startle him.

Could anybody let me know of any ideas of what/why this is happening? Is it bad news?

Ps. One of his plants (limnophila sessiliflora) had started to turn a bit brown so I have changed that for a new one.

Also, he has a waterfall filter that doesnt produces a lot of bubbles. Do you think it could be oxygen levels and would a bubble wander suffice? Aswell as a plant and filter?

Thanks guys!