Hi Keat Pek,
I've seen your webpage about lights, very good, you should be an engineer! Also you've said that you have BGA problem in your filterless office tank. Well I've got BGA before and I've learnt how to solve this problem.
Well basically BGA will appear for two reasons: Lack of nitrate(0mg/l) and/or lack of circulation(dead areas can't bring ferts to the plants).
1. If you have zero NO3 but have a good supply of other nutrients, your plants will stop growing. But BGA will grow since they can somehow fix nitrogen from the air(read bout this). Dose KNO3 until you have a NO3 value of btw 5mg/l to 10mg/l.

2. If you checked and you have sufficient NO3 in your tank, then you need to increase your circulation in that area. Maybe add a small internal filter or something. It will dissappear in its own accord.

A 3 day full blackout (cover tank with trash bags) will take care of the BGA temporarily. It will come back again if the root cause is not taken cared of.

So I would recommend that you make a 50% water change. Dose some KNO3 to the recommended NO3 amount(if you have NO3 deficiency). Then do the blackout for 3 days. After that make another 50% water change and make sure you have enough NO3 in your water. Everything should be fine then(assuming you have sufficient circulation). Don't use antibiotics or any other medicines in a planted tanks. It will do more harm then good to the plants and thus the whole 'ecosystem'

Hope this helps.