I agree with esshup but I would add that if you catch a large bass or one that looks fat throw it back. All the skinny 8-12" fish should be kept. I know it sounds weird to keep fish since we are all trained to catch and release, but a stunted bass has lost growth that it will never regain. A stunted bass might grow some more if given enough food but it will never reach it's maximum potential. Remove as many small bass as you can for two to three years and you will start to see a lot of improvement.
If there are any big bass they are eating the larger bluegill but are probably eating mostly stunted bass since they are more fusiform (easier to swallow) than bluegill or crappie.
Do a search for relative weight, learn what the optimal measurements are for these fish so you know which ones to remove. When you start catching more and more fish with good relative weights you will know that your pond is healthy.
