you sure it's not a 125? what's the last dimension.. anyway.. about a pound per gallon should work, unless you are keeping something that'd need more...?
depends on what kind of look you want you want a higher deeper looking bed or just barely covering your bottom?! Either way I would recommend atleast 100lbs for a good base.
The tank is 6'x18" and 26" high. I have 3 large angel fish and 3 small clown loaches. I want to get a few more fish but I just haven't decided what to get yet. I may re-home the angels because my husband hates them. They all reside in a 55 right now.
Thank you all for the great advice. I guess I'll get 3 bags. 2 for the tank and 1 extra for all the sand I'll be vacuuming up because I'll be a sand vacing noob.
I like a thinner bed so I went with about 60 pounds of pool filter sand in a 180 (72x24x24) gallon tank. I've found that deeper sand tends to cover and hide feces and uneaten food resulting in higher nitrates. Deep pockets can also let anaerobic bacteria develop within them, so if you do go deeper, make sure you stir the sand up during water changes to prevent that from happening.