Check your water quality for ammonia, nitrite, nitrate and pH.
Wipe the inside of the glass with a clean fish sponge.
Do a 75% water change and gravel clean the substrate.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
Clean the filter if it hasn't been done in the last 2 weeks. Wash filter media/ materials in a bucket of tank water and reuse the media. Tip the bucket of dirty water on the garden/ lawn.
When fish lay in their side and breath heavily, it's usually a bad sign.