Water quality decides whether a pond thrives or crashes. For fisheries exams — ICAR-JRF, ARS, state fisheries officer posts, BFSc/MFSc entrances — water-quality questions appear again and again, and they are easy marks if you know the standard ranges. Try these 20 exam-style questions, then check your answers below. Every answer carries a one-line explanation, so this doubles as a revision sheet.
How to use this set: cover the answers, attempt all 20, then score yourself — 16+ is exam-ready, 12–15 means revise the ranges table at the end, below 12 means read the explanations twice.
20 Water Quality MCQs with Answers
Q1. What is the minimum dissolved oxygen (DO) prescribed for fresh water for fish culture under Indian Standard IS 13891:1994?
(a) 2 mg/L (b) 3 mg/L (c) 4 mg/L (d) 6 mg/L
Answer: (c) 4 mg/L — IS 13891:1994 sets DO minimum at 4 mg/L; warm-water fish generally need above 4 mg/L, cold-water species like trout above 6 mg/L.
Q2. Which pH range is considered best for fish growth in ponds?
(a) 4.0–5.0 (b) 6.0–9.0 (c) 9.0–11.0 (d) 11.0–12.0
Answer: (b) 6.0–9.0 — pH 4–6 gives slow growth, pH 4.0 is the acid death point, and pH above 11 is the alkaline death point.
Q3. What is the optimum range of unionised ammonia (NH3) in pond water?
(a) 0.02–0.05 mg/L (b) 0.5–1.0 mg/L (c) 1.0–2.0 mg/L (d) 2.0–5.0 mg/L
Answer: (a) 0.02–0.05 mg/L — fish are very sensitive to unionised ammonia; it damages gills and blood chemistry even at low levels.
Q4. Nitrite toxicity in fish occurs because nitrite converts haemoglobin into:
(a) Oxyhaemoglobin (b) Methaemoglobin (c) Carboxyhaemoglobin (d) Myoglobin
Answer: (b) Methaemoglobin — methaemoglobin cannot carry oxygen, so nitrite poisoning is also called brown blood disease.
Q5. In nitrification, ammonia is first oxidised to nitrite by which bacterium?
(a) Nitrobacter (b) Nitrosomonas (c) Azotobacter (d) Rhizobium
Answer: (b) Nitrosomonas — Nitrosomonas converts ammonia to nitrite; Nitrobacter then converts nitrite to nitrate.
Q6. Which bacterium converts nitrite into nitrate in the nitrification chain?
(a) Nitrosomonas (b) Nitrobacter (c) Pseudomonas (d) Bacillus
Answer: (b) Nitrobacter — nitrate, the end product, is far less toxic than ammonia or nitrite and is taken up by phytoplankton.
Q7. A Secchi disc is used to measure:
(a) Water depth (b) Water transparency/turbidity (c) Dissolved oxygen (d) Salinity
Answer: (b) Water transparency/turbidity — the optimum Secchi disc visibility for fish ponds is 40–60 cm; lower visibility means heavy plankton or mud turbidity.
Q8. Dissolved oxygen in a pond is usually LOWEST at which time of day?
(a) Midday (b) Afternoon (c) Dawn/early morning (d) Midnight
Answer: (c) Dawn/early morning — photosynthesis stops at night while respiration continues, so DO falls all night and bottoms out just before sunrise.
Q9. The commonly recommended total alkalinity range for freshwater fish ponds is:
(a) 10–20 mg/L (b) 20–40 mg/L (c) 60–300 mg/L as CaCO3 (d) 500–800 mg/L as CaCO3
Answer: (c) 60–300 mg/L as CaCO3 — alkalinity buffers pH swings; IS 13891:1994 gives a tolerance range of 100–300 mg/L as CaCO3 within the same band.
Q10. Total hardness of freshwater fish ponds should be greater than:
(a) 10 mg/L (b) 40 mg/L (c) 100 mg/L (d) 300 mg/L
Answer: (b) 40 mg/L as CaCO3 — adequate hardness protects fish against harmful pH swings and metal-ion toxicity; low hardness is corrected by liming.
Q11. Free carbon dioxide in freshwater fish ponds should ideally be kept below:
(a) 1 mg/L (b) 5 mg/L (c) 20 mg/L (d) 50 mg/L
Answer: (b) 5 mg/L — IS 13891:1994 caps free CO2 at 12 mg/L at sunrise, but the desirable pond level is below 5 mg/L.
Q12. At what hydrogen sulphide (H2S) concentration do fish begin to lose equilibrium and suffer sub-lethal stress?
(a) 0.01 mg/L (b) 0.5 mg/L (c) 1.0 mg/L (d) 5.0 mg/L
Answer: (a) 0.01 mg/L — freshwater fish ponds should ideally be free of H2S; water exchange and liming (raising pH) reduce its toxicity.
Q13. What is the maximum residual free chlorine allowed in fresh water for fish culture (IS 13891:1994)?
(a) 0.02 mg/L (b) 0.2 mg/L (c) 1.0 mg/L (d) 2.0 mg/L
Answer: (a) 0.02 mg/L — chlorine is highly toxic to fish even in traces, which is why chlorinated tap water must be aged or dechlorinated before use.
Q14. Liming a pond primarily corrects:
(a) Low dissolved oxygen only (b) Low pH and low alkalinity (c) High salinity (d) High turbidity only
Answer: (b) Low pH and low alkalinity — lime raises pH, restores alkalinity and buffering, and also reduces hydrogen sulphide toxicity.
Q15. Addition of common salt (chloride) to pond water is a standard remedy for:
(a) Ammonia toxicity (b) Nitrite toxicity (c) Low hardness (d) High BOD
Answer: (b) Nitrite toxicity — chloride ions compete with nitrite at the gills, reducing nitrite uptake; aeration and biofilters prevent nitrite build-up in the first place.
Q16. How does rising pH affect ammonia toxicity in water?
(a) It decreases it (b) It increases it (c) It has no effect (d) It converts ammonia to nitrate
Answer: (b) It increases it — at higher pH and temperature, a larger fraction of total ammonia exists as toxic unionised NH3 rather than harmless ammonium (NH4+).
Q17. Fish are described as poikilothermic because:
(a) They breathe through gills (b) Their body temperature changes with water temperature (c) They migrate for breeding (d) They excrete ammonia
Answer: (b) Their body temperature changes with water temperature — that is why temperature controls metabolism, oxygen demand and growth rate in fish.
Q18. Gas bubble disease in fish is caused by:
(a) Low dissolved oxygen (b) Supersaturation of dissolved gases (c) High ammonia (d) Low alkalinity
Answer: (b) Supersaturation of dissolved gases — excess dissolved gas (for example DO above about 12 mg/L) forms bubbles in tissues and blood, and can kill fish.
Q19. The optimum biochemical oxygen demand (BOD5) value for fish ponds is:
(a) 1–2 ppm (b) 10–20 ppm (c) 50–60 ppm (d) 100–120 ppm
Answer: (b) 10–20 ppm — BOD measures the oxygen microbes need to break down organic matter; pond BOD5 typically ranges 5–20 ppm.
Q20. As per the FAO aquaponics reference table, the optimal water temperature for common carp grow-out is:
(a) 10–14 °C (b) 14–16 °C (c) 25–30 °C (d) 32–36 °C
Answer: (c) 25–30 °C — carp tolerate 4–34 °C, but grow best at 25–30 °C; Nile tilapia is similar at 27–30 °C, while rainbow trout needs cold water at 14–16 °C.
Quick-Revision Table: Standard Pond Water-Quality Ranges
| Parameter | Desirable / standard value | Source basis |
|---|---|---|
| Dissolved oxygen | Minimum 4 mg/L (warm-water fish >4; trout >6) | IS 13891:1994; FAO aquaponics table |
| pH | 6.5–8.5 tolerance; 6–9 best for growth | IS 13891:1994; standard pond criteria |
| Unionised ammonia (NH3) | 0.02–0.05 mg/L | Standard pond criteria |
| Nitrite | Desirable below 0.1 mg/L; CMFRI training manual cap 0.5 mg/L nitrite-N | Pond criteria; CMFRI Aquaculture Worker Training Manual 2020 |
| Total alkalinity | 60–300 mg/L as CaCO3 (IS tolerance 100–300) | Pond criteria; IS 13891:1994 |
| Total hardness | Above 40 mg/L as CaCO3 | Standard pond criteria |
| Free carbon dioxide | Below 5 mg/L desirable (IS max 12 mg/L at sunrise) | Pond criteria; IS 13891:1994 |
| Hydrogen sulphide | Absent ideally; 0.01 mg/L already stressful | IS 13891:1994; pond criteria |
| Residual chlorine | Maximum 0.02 mg/L | IS 13891:1994 |
| Turbidity | Maximum 10 NTU; Secchi visibility 40–60 cm | IS 13891:1994; pond criteria |
| BOD5 | Optimum 10–20 ppm | Standard pond criteria |
Why These Numbers Vary Slightly Between Books
Exact limits differ a little by species, life stage and source — for example, nitrite guidance ranges from “below 0.1 mg/L desirable” in general pond criteria to a 0.5 mg/L nitrite-N ceiling in the CMFRI training manual. In exams, answer with the value your standard textbook or the question’s source uses; the table above flags both where they differ, so you are never caught out. Ranges also shift for cold-water species (trout needs more oxygen and colder water) versus warm-water carps and tilapia.
Sources & Fact-Check Note
All ranges in this post were checked on 25 October 2026 against: Indian Standard IS 13891:1994 (Quality Tolerances for Fresh Water for Fish Culture); the ICAR-CMFRI Aquaculture Worker Training Manual 2020 (nitrite-N, total ammonia nitrogen, nitrite–methaemoglobin mechanism); the FAO small-scale aquaponics production table (species temperature, ammonia, nitrite and oxygen values); and standard pond soil-and-water-quality criteria used in fisheries teaching (pH effects, alkalinity, hardness, CO2, H2S, Secchi visibility, BOD). No deadline, scheme or funding figure is claimed in this post, so no expiry applies. The thumbnail images are illustrations, not photographs of a real pond test.