Assuming you mean exposure problems, the trouble is that you tend to shoot a shiny reflective fish against a dark (grass, earth, etc) background. The overall darkness of the composition tends to cause the camera to overexpose, which makes the fish -which needs less exposure because it's lighter generally - turn washed-out, especially the belly (chub, roach, barbel shots all tend to suffer from this) There are several solns:
1 use a camera with a spot or centre weighted exposure meter option and meter off the fish's midline area
2 shoot against a lighter background
3 underexpose by a stop or two if your camera allows it
4 "Darken highlights" in your photo editor once you've got home
Because tench are quite dark, they photograph well.
If you aesthetics/composition, I just try to add some interest like rod/reel, an open float box, or similar.
Hope this helps