This pH level is lethal for most fish.  Probably you are in the presence of extremely acidic soils.  Even if your fish are not dead, they are subjected to a difficult constraint, they will not feed well and will have virtually no growth.  You should check the alkalinity (carbonate hardness) of your water to see why your pH is so low. Adding lime to your pond will allow you to increase pH and also alkalinity.

This pH level is too low for most fish.  Your fish will be subjected to a constraint and the growth will be slow.  Prolonged exposure to this level can kill your fish.  Fish exposed to low-level pH will often have problems capturing oxygen due to the accumulation of excessive mucosal forms on the gills.  You should check the alkalinity (carbonate hardness) of your water to see why your pH is so low.  Adding lime to your pond will allow you to increase the pH and alkalinity.  You should check your pH at different times of the day as the pH changes due to photosynthesis.  You can expect the lowest pH at dawn and the highest at dusk.  If your pH is at a low level only at dawn, the increase in your alkalinity will reduce these fluctuations.  Even if a low pH level cannot directly harm your fish, it will reduce the amount of dissolved inorganic phosphorus and carbon available to plankton.  Liming will therefore increase the productivity of your water.

This pH level is ideal for fish.  Your fish will not be subjected to severe stress caused by pH problems and your water should be productive.  You should check your pH at different times of the day as the pH changes due to photosynthesis.  You can expect the lowest pH at dawn and the highest at dusk.  Make sure the pH is between 6.5 – 9 at any time of day.  If the pH appears to be very unstable, check your alkalinity.  The increase in alkalinity (by adding lime) will reduce pH fluctuations.

Your pH is higher than it should be.  Your fish will be subject to constraints.  Some species of fish can tolerate this pH level for short periods (such as late afternoon in the pond), but prolonged exposure will be fatal.  Gill damage and eye damage to the cornea will occur on most fish at this pH.

You should check your pH at different times of the day as the pH changes due to photosynthesis.  You can expect the lowest pH at dawn and the highest at dusk. If the pH appears to be very unstable, check your alkalinity.  The increase in alkalinity (by adding lime) will reduce pH fluctuations.  If this pH level is maintained for a long time, you need to check why the pH is so high.  High levels of pH can also lead to ammonia problems.  The higher the pH, the more toxic ammonia concentration increases.

Your pH is above a level that will cause death to most species.  If your fish are not dead or dying, they will be subjected to very severe constraints.  You should check your pH at different times of the day as the pH changes due to photosynthesis.  You can expect the lowest pH at dawn and the highest at dusk. If the pH appears to be very unstable, check your alkalinity.  The increase in alkalinity (by adding lime) will reduce pH fluctuations.  If this pH level is maintained for a long time, you need to check why the pH is so high.  High levels of pH can also lead to ammonia problems.  The higher the pH, the more toxic ammonia concentration increases.