Sick?

I have two adult mice that seem way too much under weight. Their sides are so small like you could practically see the ribs. I dont know why if they have plenty of food. Im considering euthunising.(excuse my spelling)

Did you just get them? They could either be underfed from before, or maybe they have some parasites.. did you notice any missing fur or scabbies on them, or wetness/yucky coloration under their tails?

Actually now that you mention it they did have some wetness under the tail. I though it was urine and their coat wasnt to good looking either.

Check their water.

While Ginevive gave you some great suggestions this is one that often gets missed.

Every now and then water bottles get stuck closed and the animals are unable to get water ... they exhibit the same signs that you stated.

From my own experience I've noticed that this only happens to my mice when a drinking bottle or valve gets stuck closed.

I fill water bottles all on the same day leaving about an inch of air in the bottle to create less vacuum, I can then tell at a glance if one enclosure is consuming less water.

Bryan

Well it was two that were looking sick. One wasnt so bad looking just skinny, i fed it off. The second looked better in terms of weight, this is the one with the wetness under the tail, and just now i went to check and she was dead. I was keeping 3 females to 1 male. 2 females are out and in this same cage a litter was born today.

Hm.. I would consider your source, if you recently got them.. were they in an overcrowded cage? They may have had to fight over food/water.. Also I would Google "wet tail" and see if they had those symptoms.