Fish keeping and breeding is a science. Each breed of fish will have a pH that is optimum for survival and breeding. This pH will be different and possibly unique so care needs to be taken when mixing breeds in the same pond or tank. Ponds are generally freshwater and cold whilst aquariums can be either freshwater or saltwater. In all cases pH will need to be measured along with salt concentration. Nitrate levels should be monitored to ensure healthy water life is maintained.