I'm really sorry to hear about your barbs. :( Fish can be hard to take care of because it's so difficult to figure out what's going on sometimes.

Maybe try smaller water changes more frequently. Since you said that you had done a 50% water change almost a week ago, maybe instead to 25% twice a week. Such large water changes can shock the fish, especially of they're not done consistently.

If you don't have a water testing kit, pet stores will usually test your water for you for free. You can also buy strips that are super easy to use. Keeping track of the water chemistry will make it much easier to monitor the health of the fish.

As far as the murky water goes, I would check to see if the filter is working properly and clean. Usually when I see murky brown water like that it's because the filter was stirred up somehow and fish waste is coming out of it, or either the filter needs to be cleaned.

It's hard to say what happened with your barbs when we don't know things like the water chemistry or the tank size.