First, I might suggest you add more LR. We usually aim for +/- 1/8kilo per litre so you could have 6 kilos in there or more. You really could even got to 9 or 10 kilos just depending on how you want your tank to look. The LR is the BEST form of filtration that there is in this hobby, so having the proper amount will be a big benefit to you. As far as the nem goes, they usually don't start to look unhealthy until it's too late and unfortunately, they die slowly. By the time they look bad, their flesh is often too week to handle and when you try to remove them they fall apart. It often leads to a complete tank crash. As stated above, if it is moving around the tank, that is natures way of telling you that there is a problem that needs to be addressed. It's either not getting enough light, or the parameters are too unstable. What kind of lights do you have? You said that your temps are between 25-29*... I'm not sure if you mean that they are changing that much, but if so, that is a problem. 4 degrees is too much variation and it will cause constant stress in your livestock. Your target temps should not exceed 25*min and 27*max but even more importantly, you should be focused on stability. On the Fahrenheit scale, 1/4* to 1/2* is about all that we let them move. In a small tank, temperature stability is probably the most difficult thing to maintain. I'd look at this and your lighting as possible sources of the problem. I'd also recommend that you start doing smaller water changes weekly as that will also help minimize parameter swings in your tank.
