Hello and thank you for your message,
Without knowing all the details of your tank (size, water parameters, lighting, filtration, etc...) it is hard to give exact answers but I can give some general type answers.
It is fairly normal for a tank to go through different cycles during the first year or so where different algae types will thrive more than others. I have linked a couple of articles below about algae that hopefully will help some:
As far as why your fish died that is hard to say. They may have had some type of disease, or were not healthy to begin with. If it was a water quality issue, then doing some water changes can help out. If they were the only fish in the tank I would suggest not adding any new fish for at least a month in case it was some type of disease.
I would recommend sending an email directly to us at email@example.com with a little more information about your tank and from there we can hopefully troubleshoot a little better for you.
Marine Depot Customer Service