20" joists ?, wow, I only got 10" and had to double them up (8" on centers now).
My joists also run leghtwise to the tank, I had to build two of these:

Never mind the white one, that's from my current tank (150 gal).
I'm glad you got it figured out, you can never be too safe and there is no such thing as overbuild. I tend to do the same.
I'm looking forward to your reports.
We should I guess ask the mods to move this to the Tank Journal forum.
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Denis
150g SPS reef;3x250WHQI,Tunze Streams,Wavebox,Neptune Apex,fishroom in basement

,reefing since 93,SPS since 99.
TOTM June 2010