Effects of pH on fish and aquatic life

The balance of positive hydrogen ions (H⁺) and negative hydroxide ions (OH^-) in water determines how acidic or basic the water is. Notice the ' ⁺ ' and ' ^- ' in the chemical symbols above. They indicate that these chemical forms are 'ions' — they have a positive or negative electrical charge. This means the molecule in question is either missing an electron or has an extra electron. Since electrons have a negative charge, an extra one in the OH molecule makes it OH^-, and a missing one in the H molecule gives it a "missing-minus" charge — in other words, positive — and makes it H⁺. When analysts measure pH, they are determining the balance between these ions.
(to remember what pH is, think of the term "pH" as positive Hydrogen).
The pH scale ranges from 0 (high concentration of positive hydrogen ions, strongly acidic) to 14 (high concentration of negative hydroxide ions, strongly basic). In pure water, the concentration of positive hydrogen ions is in equilibrium with the concentration of negative hydroxide ions, and the pH measures exactly 7.
In a lake or pond, the water’s pH is affected by its age and the chemicals discharged by communities and industries. Most lakes are basic (alkaline) when they are first formed and become more acidic with time due to the build-up of organic materials. As organic substances decay, carbon dioxide (CO₂) forms and combines with water to produce a weak acid, called "carbonic" acid — the same stuff that’s in carbonated soft drinks. Large amounts of carbonic acid lower water’s pH.
Most fish can tolerate pH values of about 5.0 to 9.0, but serious anglers look for waters between pH 6.5 and 8.2. The vast majority of American rivers, lakes and streams fall within this range, though acid rain has compromised many bodies of water in our environment.
Synergistic Effects of pH
Synergy is the process whereby two or more substances combine and produce effects greater than their sum. For example, 2 + 2 = 4 (mathematically). But synergistically, 2 + 2 = much more than 4! Synergy is a mathematical impossibility, but it is a chemical reality. Here’s how it works:

When acid waters (waters with low pH values) come into contact with certain chemicals and metals, they often make them more toxic than normal. As an example, fish that usually withstand pH values as low as 4.8 will die at pH 5.5 if the water contains 0.9 mg/L of iron. Mix an acid water environment with small amounts of aluminum, lead or mercury, and you have a similar problem—one far exceeding the usual dangers of these substances.

The pH of sea (salt) water is not as vulnerable as fresh water’s pH to acid wastes. This is because the different salts in sea water tend to buffer the water with Alka-Seltzer-like ingredients. Normal pH values in sea water are about 8.1 at the surface and decrease to about 7.7 in deep water. Many shellfish and algae are more sensitive than fish to large changes in pH, so they need the sea’s relatively stable pH environment to survive.
Shallow waters in subtropical regions that hold considerable organic matter often vary from pH 9.5 in the daytime to pH 7.3 at night. Organisms living in these waters are able to tolerate these extremes or swim into more neutral waters when the range exceeds their tolerance.
 

Table 5. Effects of pH on fish and aquatic life
pH value		Effects observed under research
Min	Max
3.8	10.0	Fish eggs could be hatched, but deformed young were often produced.
4.0	10.1	Limits for the most resistant fish species.
4.1	9.5	Range tolerated by trout.
4.3	--	Carp died in five days.
4.5	9.0	Trout eggs and larvae develop normally.
4.6	9.5	Limits for perch.
5.0	--	Limits for stickleback fish.
5.0	9.0	Tolerable range for most fish.
--	8.7	Upper limit for good fishing waters.
5.4	11.4	Fish avoided waters beyond these limits.
6.0	7.2	Optimum (best) range for fish eggs.
1.0	--	Mosquito larvae were destroyed at this pH value.
3.3	4.7	Mosquito larva lived within this range.
7.5	8.4	Best range for the growth of algae.

Industrial processes that use water can be affected by the pH level, and in many instances the pH is adjusted by adding chemicals or buffers. The table below shows optimal pH levels for a few different industrial processes.

Table 6. Optimal pH for industrial water supplies
Process	Minimum	pH Range
Food canning and freezing	7.5	--
Washing clothes	--	6.0-6.8
Rayon manufacturing	--	7.8-8.3
Steel making	--	6.8-7.0
Tanning leather	--	6.0-8.0