First and foremost - No, it isnt bad for your fish. Its just algae
Its BBA (Black Beard/Brush Algae). Its a pain to get rid of once it sets in. As your well aware there are millions of types of algae so sooner or later we all encounter one or another. I myself have this in one of my tanks, its been there for about 3 years now as i've struggled to get rid of it.
Heres a quick extract from James Algae Guide (from UKAPS) on BBA
"
Causes
In a high light tank it is an indication of low or fluctuating CO2 levels or not enough water circulation around the plants. In a low light tank it is often due to changing CO2 levels.
I would hazard a guess its down to fluctuating CO2 levels, this seems to be the most common cause of BBA. As for removing it, i'm afraid its a pain as i have already said. Manual removal is your best bet, atleast as much as you can because its really really stubborn. Even the best algae eating fish/shrimp/plecs seem to leave BBA as its just to stubborn. Try getting it off with tweezers and you'll soon see.
Personally i think your best bet is to take out the affected decor, anything but the gravel basically and boil it in a big pan. This is what i did and it kills off the algae. Once its boiled i gave it a good jet spray with a car jet wash kit (if you have one). A shower head works well also
Gravel your just going to have to do what you can with it. I chuck away any gravel grains that have become big tufts of BBA as its not worth the hassle, i suppose you could boil this also, its just more time consuming and harder to separate dead BBA from the rest.
I noticed you said your getting Seachem Excel. This is said to be good at getting rid of BBA, although precise dosing is required. Usually lowering the water level so the infected area is above the water surface and directly applying the Excel to the BBA (using a pippette ideally). This in practice can be hard to do if its mainly on the bottom of the tank that is infected.
They also say a 'blackout' of the tank can assist in removal of BBA. Don't do it, fools belief. I'ver never found this effective even in the slightest. Its just too stubborn
As for preventing it from future occurence you need to address the fluctuating CO2 levels. If you don't dose CO2 in any form at all then your levels are going to fluctuate massively each and every day, and during every water change as you go from high-low-high-low ect. Contrary to some belief that BBA thrives where water movement is best (never happened to me). Mine always takes hold in circulation deadspots. Positioning of your inlet/outlets is key to circulation. Getting it perfect is near impossible, but getting it close will help. You could get a powerhead to help with flow, but i don't think these are essential unless your going into a serious planted tank with priority over plants rather than fish
From what you've said your light levels arent the issue, so i wouldnt worry about this. You could up your lights to a max of 10 hours a day without any issues. Personally, i always go for 8 hours as a benchmark.
Hope this helps,
James
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