wrote in message ups.com...
I did this and you are 100 % correct - Thanks,

Now any advice how I can get rid of that chunky alagee that formed in
the substrate due to the heat rise?

I did do a 20 gallon water change but the sand top keeps turning a dark
purple-ish - green.

How long was your tank running without ligt after your lamp broke ?
It had acumulated some waste/nutrients and it needs time to cycle
them into the algae bodies...
Make sure your grazers crew is alive. Check phosphates/nitrates.
20 gallon water change for a 120 gallon tank is a weekly maintenance.
In case of emergency or big waste buildup you need more massive
water changes. Check the levels of phosphates/nitrates and keep them
low: temporarly with water changes, later - long term with grazers
feeding on algae growing in your tank.
Do not forget the live fish tank is a very dynamic system and big
change in lighting will need proper time to reach new equilibrum.