In 30 years of fish-keeping I have never seen this.Avoid white sand though as most fish prefer darker sand, and look better against it.
Yes, barring the fish can handle lower KH and pH around 6.5 ish-- aquaspoil is entirely safe for fish. A big HOWEVER is that you must wait for leaching to stop. \
It is white substrate that is the problem. This does affect many fish because they do not expect bright but dark substrates. It becomes even worse under the tank lighting which is usually far brighter than what the fish experience in nature as they tend to come from forest watercourses that are often well shaded.In 30 years of fish-keeping I have never seen this.
Fish have a preference for substrate color?
Ill tell that to my wild discus and wild eartheaters who are on white ( now slightly tan) sand.
This color is all wrong for you- lol!
Home Depot, Lowe's both carry Quikrete Play Sand. Depending where you live in Canada, you may find the dark grey mix (it is here in Vancouver area) or the buff tone (looks more like ocean beach sand). I do prefer the dark grey but either is fine. I contacted Quikrete some time back and they were very helpful detailing how they refine this sand so it is the safest sand you can get.I should also ask: is there such thing as dark or black play sand?? In Canada, not certain where to find such a thing, especially this time of year. ?