Freshwater Fish Care
Water Parameters by Species
The most common cause of fish health problems is a mismatch between water parameters and species requirements. This guide gives you the exact pH, temperature, GH, KH, and tank size requirements for every major freshwater fish group.
By Hasan, Aquascaping Lead & Species Specialist, AD Aquatics · Updated 2025
Master Water Parameter Table
Use this table to check whether your target species are compatible with each other and with your local tap water. Check our UK tap water hardness guide first to know your starting water parameters before choosing fish.
| Species Group | pH Range | Optimal pH | Temp (°C) | GH (°dH) | KH (°dH) | Min Tank (L) | Bioload | UK Tap Suitability |
|---|---|---|---|---|---|---|---|---|
| Community Tropicals Tetras, rasboras, danios, barbs, gouramis | 6.0–7.8 | 6.8–7.2 | 22–28 | 4–15 | 2–10 | 60 | Low–Medium | ✓ Most UK regions suitable; soften if GH >18 |
| Corydoras & Catfish Corydoras, Otocinclus, Bristlenose pleco | 6.0–7.8 | 7.0 | 22–26 | 2–12 | 1–8 | 60 | Low | ✓ Most UK regions; avoid very hard water (GH >20) |
| Livebearers Guppies, platies, mollies, swordtails | 7.0–8.5 | 7.5–8.0 | 24–28 | 10–25 | 8–18 | 60 | Low–Medium | ✓ Excellent — thrive in most UK hard tap water |
| African Cichlids Mbuna, Peacocks, Haps (Malawi); Tanganykians | 7.8–8.6 | 8.0–8.3 | 24–28 | 12–25 | 8–18 | 150 | High | ✓ Ideal for hard UK water; may need hardness boost in soft regions |
| South American Cichlids Oscars, Severums, Jack Dempseys, Geophagus | 6.5–7.8 | 7.0–7.2 | 24–28 | 5–15 | 3–10 | 200 | Very High | ✓ Mid-hard UK water suits most; avoid extreme hard (GH >20) |
| Discus Symphysodon spp. | 5.5–7.0 | 6.0–6.8 | 28–32 | 1–8 | 0–3 | 200 | Medium | ✗ Most UK tap water too hard — RO required in virtually all regions |
| Softwater Tetras & Rasboras Cardinal tetras, rummy nose, harlequin rasboras | 5.5–7.0 | 6.0–6.8 | 24–28 | 2–10 | 0–5 | 80 | Low | △ Soft UK regions (Wales, NW England) suitable; hard regions need RO blending |
| Goldfish & Coldwater Common, fantail, shubunkin, koi (indoor) | 6.5–8.0 | 7.2–7.6 | 10–22 | 8–20 | 5–15 | 200 (single fish) | Very High | ✓ UK tap water generally ideal; no heating required |
GH = General Hardness (total mineral content). KH = Carbonate Hardness (buffering capacity). ✓ = suitable without treatment in most UK regions. △ = partially suitable depending on region. ✗ = water treatment required. See UK regional water data to check your specific supply.
Species Compatibility Matrix
Mixing species from different groups requires water parameter overlap AND compatible temperament. This matrix shows which groups can share a tank. "Possible" requires the keeper to verify individual species requirements within each group.
| Species Group | Community Tropicals | Corydoras | Livebearers | African Cichlids | South American Cichlids | Discus | Softwater Tetras | Goldfish |
|---|---|---|---|---|---|---|---|---|
| Community Tropicals | ✓ | ✓ | Possible | ✗ | ✗ (size risk) | Possible | ✓ | ✗ (temp) |
| Corydoras | ✓ | ✓ | Possible | ✗ | Possible (small SA species) | Possible | ✓ | ✗ (temp) |
| Livebearers | Possible | Possible | ✓ | ✗ (aggression) | ✗ | ✗ (pH conflict) | ✗ (pH conflict) | ✗ (temp) |
| African Cichlids | ✗ | ✗ | ✗ | ✓ (same lake) | ✗ (pH conflict) | ✗ | ✗ | ✗ |
| South American Cichlids | ✗ | Possible | ✗ | ✗ | Possible (similar size) | ✗ (temp) | ✗ | ✗ |
| Discus | Possible | Possible | ✗ | ✗ | ✗ | ✓ | Possible (softwater) | ✗ |
| Softwater Tetras | ✓ | ✓ | ✗ | ✗ | ✗ | Possible | ✓ | ✗ |
| Goldfish | ✗ | ✗ | ✗ | ✗ | ✗ | ✗ | ✗ | ✓ |
✓ = generally compatible · Possible = compatible with care (verify individual species) · ✗ = incompatible. "Same lake" for African cichlids means mixing Malawi with Malawi species or Tanganyika with Tanganyika species — never mix lake species from different lakes.
Bioload Estimation Guide
Bioload determines filter sizing and stocking density. The old "1 cm of fish per litre" rule is outdated — it fails to account for body mass, metabolic rate, and waste production. Use bioload categories instead:
| Bioload Category | Examples | Stocking Guide | Filter Turnover | Water Change Frequency |
|---|---|---|---|---|
| Low | Small tetras, corydoras, rasboras, Otocinclus | Up to 1 fish per 5–8 L (small species) | 5× tank volume/hr | 25–30% weekly |
| Medium | Gouramis, discus, medium tetras, platies | 1 fish per 10–15 L | 6× tank volume/hr | 30–40% weekly |
| High | African cichlids, large barbs, angelfish | 1 fish per 15–25 L (species dependent) | 8× tank volume/hr | 40–50% weekly |
| Very High | Oscars, goldfish, large South American cichlids | 1 large fish per 50–100 L | 10× tank volume/hr | 40–50% twice weekly |
Species-Specific Care Notes
Discus
Discus are the most demanding freshwater species commonly kept in the UK. Beyond water chemistry, they require:
- Temperature: 28–32°C (significantly higher than most fish — plantmates must tolerate warm water)
- Water changes: 30–50% every 2–3 days to keep nitrate below 20 ppm
- Group size: Minimum 6 — discus are schooling fish and develop stress-related diseases when kept in pairs
- Tankmates: Cardinal tetras, Sterbai corydoras, and Altum angelfish tolerate discus temperatures; avoid most other species
- RO water: Required in virtually all UK regions — most tap water exceeds their GH/KH tolerances
See our dedicated discus fish care guide for full husbandry detail, RO mixing ratios, disease identification and feeding schedules. Our Discus Planted setup package starts from £1,650 fully installed.
African Cichlids
African Rift Lake cichlids are ideal for most UK hobbyists — they thrive in hard, alkaline water that matches or exceeds typical UK tap water parameters. Key considerations:
- Lake compatibility: Never mix Malawi and Tanganyika species — their chemistry overlaps but behavioural and dietary needs differ significantly
- Aggression management: Overstocking (counterintuitively) reduces aggression by distributing territorial behaviour — a 200-litre Malawi tank should hold 15–20 mbuna, not 5–6
- Rocks over plants: Most African cichlids destroy plants. Use Anubias tied to rocks for any greenery
- Protein warning: Mbuna are predominantly herbivorous — high-protein foods cause Malawi bloat, a fatal digestive condition
See our African cichlid care guide for stocking lists by lake and our Cichlids Setup package from £650.
Planted Tank Species
Heavily planted tanks suit softwater species (tetras, rasboras, corydoras, discus) best — plants prefer slightly acidic conditions that overlap with these fish. Avoid African cichlids and most livebearers in serious planted setups. See our planted aquarium service for CO₂ and plant selection expertise.
Frequently Asked Questions
What water parameters do discus need?
Discus require soft, warm, acidic water: pH 5.5–7.0, temperature 28–32°C, GH 1–8°dH, KH 0–3°dH. They are the most demanding species in freshwater fishkeeping and require RO water in virtually all UK regions. Nitrate must be kept below 20 ppm, requiring water changes every 2–3 days. See our discus fish care guide for full detail.
What water do African cichlids need?
African Rift Lake cichlids (Malawi, Tanganyika) need hard, alkaline water: pH 7.8–8.5, temperature 24–28°C, GH 12–25°dH, KH 8–18°dH. This closely matches most UK tap water, making them one of the easiest groups to keep in the UK without water treatment.
Can I mix tetras and cichlids?
Only in certain cases. Community tetras can coexist with South American dwarf cichlids (Apistogramma, German Blue Ram) at similar pH 6.5–7.2. They cannot share a tank with African cichlids (pH conflict) or large South American cichlids like Oscars (predation risk). Discus-specific tetras like cardinal tetras make excellent discus tankmates.
How many fish can I keep in a 200-litre tank?
Stocking depends on species bioload, not just a number. A 200-litre community tank might hold 30–40 small tetras/rasboras, or 12–15 medium fish (gouramis, medium barbs), or 10–12 African cichlids (mbuna), or 5–6 discus, or 1–2 Oscars. Use the bioload table above to estimate for your chosen species.
Not sure which fish suit your water?
AD Aquatics will test your tap water and recommend the best species for your local supply — or specify the right treatment to open up softwater species.