New Rouen ducklings 3 days old

Just got 4 Rouen ducklings, 1 male 3 females. Was advised to keep them in a 10gal aquarium with a 75 watt light over one end for warmth. Have added the Sav-a-chick probiotic and electrolyte to a gallon of water which I mixed then put into a quart drinking container for them.I also have a small dish with regular water. I have also added 500mg of Niacin to the water as a supplement.

I have red cedar wood shavings mixed with pine wood shavings as a base for them. Have a thermometer in the aquarium that shows 75 degrees roughly. I put some corn mash in for them given to me by the breeder. However they seem to be eating the wood chips?? Is this good for them? They don't seem to be touching the corn mash. Correction now they seem to be finally eating the mash. For a while they were all huddled together towards the center until I put the corn mash in then they all got active doing the above.

I have a 4'x8' tractor house I am building for them which should be done within a week or two. So housing won't be an issue. Already got some ideas off here and got several books. Though this is my first adventure into ducks. Only fish, dogs, cats and horses, prior or current.

Any suggestions, things to watch out for? Things I am doing incorrectly?

Planning on changing the bedding fairly frequently. Don't have a lot of water in at least nothing they can put their whole head in. Going to fill the dish up I put water in just a small one. But won't do that till tomorrow when I can keep an eye on them in case they get in the dish.