Looks spot on to me buddy
What fish have you got in there I can see neons?
Your gravel patches will be all over the tank and beneath the sand eventually, consider some small rocks or slate as dividers, to DELAY it. Also, top it off! Maybe its just me being OCD, but a low water line drives me crazy!!! Other than that, looks great.
Dont they grow too big for my tank? Its around 27gallonCory's will love your tank
No mines only 20 gallon (3ft) long and I have four in and they love it they may mix your sand because the constantly sift through looking for food, have a look on google they are called corydoras I have peppered ones but there are loads of different species
Just so you know the correct term, cory's are a shoaling species, not schooling. Shoaling fish enjoy the social company of their own species and will often forage and hang out in a group but they also will happily go about on their own when they want.