How much gravel?

It's all personal preference really, some use two inch coverage some go by less. I use very little about 1-1/2 inch coverage for my 90gal. some people do not use any at all. if your going to use plants you might want a little more.
 
If you're not going to use real plants, a very thin layer of sand (1/2"-3/4") really takes a load off of you maintenance wise. However if you're looking at a planted tank, at least 1 1/2" of gravel will do, most plants wouldn't need more than 2".

good rule of thumb is 1 lb per gallon, it should get you about right.

I'm not sure that would work very well in a hex tank, usually they're quite tall and not very wide or long.
 
oops, totally looked over the Hex part, sorry, just saw 35 gallon. my bad.

Yeah, it depends on the dimensions. Lots of hex tanks are custom made.
 
yeah, just missed the hex part, works pretty good on long tanks for about 1 1/2 to 1 3/4" of gravel, lil less on the talls and hex's and more on the wides
 
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