Hello,
I know how you feel regarding BBA
What you need to do as the first step is to double-check your CO2 levels.. and double-check it again. Is it stable? As in, is your drop checker always a green colour during lights-on. Plants require, therefore MUST HAVE, as close to as a stable CO2 level as possible - major fluctuations in CO2 levels can have a negative effect and benefit algae growth.
Next.
Seachem Flourish EXCEL.
Using this is much safer than H202 and/or bleaching!
This is a great product. This kills off all BBA GUARANTEED. The best way to administer this is via a syringe (just a normal plastic one is fine).
1) Turn off filter and all water flow.
2) Fill syringe with Flourish Excel (dependent on tank size - read label).
3) Squirt the Flourish Excel SLOWLY over the affected areas where BBA has taken over.
4) Wait 10 minutes making sure there is no flow in the water - i.e. the water is still.
5) Power up all your filter(s) and pumps.
Then, within 24-72 hours (depending on how badly the BBA is), the BBA should turn from dark green/black to blue/purple/red. Then it will disappear.
According to many sources, BBA is caused by low levels of CO2 or fluctuating CO2 levels. Just make sure you are pumping a constant level of CO2 into the tank (i.e. the drop checker is a lime green). Obviously tanks don't require CO2 during night-time.
Also, give your plants plenty of nutrients - try and dose using the EI method if you can, this is the best way to dose your tank and has worked extremely well for me. And for the lights, well, definitely no more than an 8 hour period.
One final note. The Seachem Flourish Excel is not a permanent fix to your BBA problem. The BBA may come back if you do not get the balance between CO2, nutrients and light correctly. Also, continue to dose Flourish Excel daily as well as using pressurised CO2 with a decent regulator.
And the one thing that people always think is not important to do..
WATER CHANGES! - Trust me, waste products in the water (e.g. Ammonia) favours algae growth. Do a weekly water change (50% if dosing the EI way for nutrients) and that'll help a lot!
In regards to the Molly you are thinking of adding.. you won't notice any major differences for a while. Try the above first.
Best of luck,
Dan