Yes, most aquarium lights mimic sunlight to help plant growth and algaes are simple plants. Try changing it to a 4 hours on, 4 hours off cycle.
On a separate note, my calculations give your volume at around 3.5gallons. That is too small for a 6"+ goldfish. They need 8-10gal per fish. That might be part of the reason why you are having an algal problem. Try to get a bigger tank for your fish if you want it to be healthy and happy.
Well, 0.25-0.5 is really not a spike. It is more a bump than a spike. I doubt it would have done much harm to the fish in the short time they are exposed to it unless the water is really high in pH but that is another story. They might be a bit annoyed by it but that should be about it.
As for why? That is a hard question to answer without intimate knowledge of your setup. Your filter might be borderline in handling your fish load, you might have overfeed them a bit, there might be a dead snail/fish somewhere. A minor glitch with the filter as you work with the nitrazorb, etc, etc. It could be a number of things but as it is only a small rise, it will be hard to trace unless it won't come down and continue to rise.
Are you going to keep any fish in the pond? If yes and they are of any decent size then it is very hard to keep the gravel in place. They will pick them up and split them out all over the place. There is also a question mark over whether gravel is good for pond health.
That does sound very serious. If the gills looked inflamed and sticking out, it could be bacterial gill disease. Not easy to treat. Lying on her side is not a good sign. Is she eating anything at all? Any attempt to swim if you approaches the tank?
Hmm, sounds like internal bacterial infection. Can you run a check of her gills for me. Do they look fresh meat red or pale pink/grey/brown? Are they in a nice fan shape, not slimey/tattered? Any blood spots under the skin? Gasping?
Interpet #9 is usually recommended but please check that it is okay for tropicals as well. It will be more challenging to keep the goldie with tropicals. Good luck!
While I have 5 tanks in my house, my broader fish health exposure comes from ponds and koi's. Most koi keepers do not believe in general tonic type of additives because most ponds are open systems will a complex ecosystem so keeping the water good paramount but not really in the "crystal clear" way. Any type of additives are looked at with suspicion and skepticism (you would too if you work out how much it will cost to dose a 2000 gal pond! ) There is certainly a lot less tickering with the water. But having said that, they would PP a pond with not a second thought or use Quickcure if they think it is justified.
Unfortunately, a lot of manufacturers of such additives are exploiting peoples wish to give their pets the best regardless of whether they need it or not. I'm sure that a lot of them will not harm the fish but are they really necessary? Seeing no negative reaction doesn't mean that it is doing good either especially if the fish is healthy in the first place.
As you know, their gills are the equivalent to our lungs. Within the gill structure itself, there is only 1 to 2 cell's depth between the water and their blood stream. That is how the oxygen transfer from the water to their blood (red blood cells). What we don't want is to coat that extremely thin and delicate structure with anything at all. No more than us wanting to coat our lungs with aloe vera! On top of this, the fish is equipped by nature to generate their own protective coating (their slime coat) and it is indeed a question mark whether paying the extra cost of the additives are actually helping the fish or giving us a "peace of mind" when none is required or desired. BTW, most dechlor products shouldn't contain aloe. The only one that I'm aware of is stresscoat (and you pay a premium for it) but I'm not familiar with all the products available in the UK.