What is wrong with my ducklings?

I have 2 ducklings, they are around 16 days old today, and I have had them for about a week and a half. They are two different kinds, but I don't know exactly what. The one that is a little bigger is all yellow with pink feet and bill, and the other is 2 shades of brown, and has black feet and bill, like a mallard. The yellow one looks like it's skin is kind of red underneath it's fluff, and the brown one doesn't stand much and when I took it outside today, it seemed like it really didn't want to walk, and when he did, he would hold his head way in front of him and run. Also, for the past couple days they haven't been eating and drinking as often as they were before. every day I weigh them, and usually they would gain 1/3-1/4 of their weight every day, but the brown one weighed 310g yesterday, but today he weighs 340 I think. I also noticed that they sneeze a lot, and sometimes their poop is really wet and runny. For most of the day they are kept in a large plastic storage container with a light for heat. I try to maintain a temperature of between 80 and 90 degrees F now, and they are fed chick feed from a plastic lid from a jar, and I try to keep it filled. Their water is in a water dispenser, which I also try to keep filled. I feel bad that they have to grow up in captivity, and my plan is that in a couple weeks I will let them live outside in a large pen I will make in my yard over the weekend from metal fencing, and put the plastic box as a sleeping shelter. After that, I might let them go at a well populated duck pond, so hopefully people can feed them, and they can learn from other ducks. Can anyone tell me what I am doing wrong? I just want them to have a decent life.