joannde,
High phosphates have no role in BGA. My tank (if I trust my liquid test kit) is probably over 4ppm at any given time. I have never had BGA or any other algae because of phosphates.
Excel will not get rid of BGA, it just ensures your plants have a constant carbon source. If you have other nutrient deficiencies, the Excel will not help.
I do not use antibiotics in my tank, but in your situation, if higher levels of nitrAtes and PWC's and good ferts (including CO2) do not rid your tank of them I would consider using the antibiotics.
Antibiotics are safe WHEN USED PROPERLY. Most times they are used for a day or two, the person see's positive results (algae/bacteria dying), and then stops. This is how you get resistant strains. If you've ever recieved a prescription for a bacterial infection, you are always told to take the FULL course of medicine regardless of how well you feel after a day or two. This ensures that ALL of the pathogen is killed.
What happens when only a partial dose is used is some of the pathogen might just barely survive. If it has come in contact with the antibiotic, it can develop an immunity towards it and now that antibiotic is ineffective against that pathogen. This is very dangerous in humans, but also in tanks since you now have to use a different antibiotic. This unfortunate circle continues if the procedure is not properly followed.
Please note it is also possible that your BGA is ALREADY resistant or immune to the antibiotic you are going to use.
Slight rant:
I highly recommend NOT using antibacterial soaps/creames in your household. I also highly recommend AGAINST having your children use these soaps as well. It is very VERY easy to get a resisitant strain to many different antibiotics due to the frequency and sometimes poor washing techniques that many children (and adults) have. Back in college I was utterly shocked when I took a water sample from a pond in my parents backyard. We isolated a single bacteria and then tested it against about 15 commonly used antibiotics (many which are found in handsoaps/etc). The random bacteria I had tested was resistant to 6 and completely IMMUNE to 3 of these. This was a bacteria that should have had very little contact with anything, and goes to show just how dangerous careless use of antibiotics can be. I forsee a time in the near future when a deadly strain of a previously nuisance organism develops immunity to a large amount of antibiotics. Frankly I'm surprised it hasn't happened yet.
/end rant