New ponds
Clean out excess weed (not all the weed).
Replant old & overgrown weeds & plants.
Use aeriating weeds
Introduce Oxygen to the water, this will help start the cycle.
Sludge is a killer, it harbours aeromonas. drag it out.
Provide cover for you fish to hide(lillies etc)
introduce a natural form of aeriation such as a waterfall
Power it by a pump to suit the size of the pond.
Provide a filter if you are going to put in fish of more than 1" to a square foot of surface area.
Examine your fish for external problems such as missing fins or green patches.If you have any of these examine the treatments available.
This is just the start, and will rectify any or all of the problems that you have.
Regards
www.fishkeepingsupplies.comAquarium and Fish Supply Tropical