Freshwater Water Parameters, By What's Actually Living In Your Tank

Freshwater is trickier to give blanket advice on than reef, honestly, because "freshwater" isn't one target — a shrimp tank, a planted tank, and a community fish tank all want genuinely different numbers, and treating them as the same thing is how people end up with a tank that's fine for their fish but starving their plants, or dialed in for their plants but killing their shrimp. So instead of one universal target, here's what each actually wants, and the biology behind why.

Plants

Plants care less about pH itself and more about what pH does to nutrient availability — push it too high and iron and other micros lock out of solution where roots and leaves can't use them anymore, even if the nutrient is technically still in the water.

The clearest example is carbon. Dissolved inorganic carbon in water exists as CO2, bicarbonate, and carbonate, all in one pH-dependent equilibrium, and as pH climbs past about 7, that balance shifts hard away from free CO2 and toward bicarbonate. Most aquatic plants can only take up carbon as CO2 directly through their leaves; only some species carry the enzyme carbonic anhydrase, which lets them pull carbon from bicarbonate instead by converting it back to CO2 right at the leaf surface. That's the actual mechanism behind why a high-pH planted tank can stall out even with plenty of fertilizer in the water — the carbon simply isn't in a form most of the plants can use. Iron runs into a similar problem: as pH rises, dissolved iron increasingly oxidizes into an insoluble ferric hydroxide precipitate, which is exactly why iron supplements are chelated — bound to an organic molecule like EDTA or DTPA — to keep it in a plant-available form long enough to actually get absorbed.

• Temperature: 72–82°F covers almost everything sold as a beginner-to-intermediate aquatic plant.
• pH: 6.5–7.5. Slightly acidic is generally friendlier to nutrient uptake than alkaline.
• GH: 3–8 dGH. Plants actually want some calcium and magnesium in the water — don't run it too soft on their account.
• KH: 0–6 dKH. Low is fine and often preferred; high KH is what drags pH up and starts locking out micros.
• CO2: ambient is enough for a low-tech tank growing easy stuff like Anubias and Java Fern. If you're running a high-tech planted tank with demanding stem plants, injected CO2 targeting roughly 20–30 ppm is where real growth shows up.
• Nitrate: plants actually consume it as a nutrient, so some — 5–20 ppm — is a good thing, not a problem to chase to zero.

Fish

This is the category where "it depends on the species" is doing the most work, so treat these as a starting frame, not gospel for every fish you'll ever stock.

Freshwater fish are fighting osmosis constantly. Their blood and tissues are saltier than the water around them, so water is always diffusing into their body through their gills, and they have to actively pump ions in and excrete large volumes of dilute urine just to keep from swelling. That ion-pumping machinery is exactly what nitrite interferes with. Nitrite gets absorbed through the same gill channels fish use to actively import chloride, and once it's in the bloodstream it oxidizes hemoglobin into methemoglobin, a form that can no longer carry oxygen — fish pathologists literally call the resulting condition "brown blood disease." It's also the actual reason freshwater fish tolerate so much less nitrite than marine fish do: marine fish already live in chloride-rich water and aren't relying on the same aggressive ion-importing gill channels that give nitrite an easy way in.

• Temperature: 76–80°F for most tropical community fish; 65–72°F for coldwater species like goldfish and White Clouds.
• pH: soft-water South American fish (tetras, discus, Apistogramma) want 5.5–7.0; general community fish do fine at 6.5–7.5; livebearers and African rift lake cichlids want it pushed up to 7.5–8.5.
• GH/KH: mirrors the pH split above — soft for the acid-loving South American species (GH 2–8, KH 0–4), noticeably harder for livebearers and rift lake cichlids (GH 10–20+, KH 8–15+).
• Ammonia and nitrite: 0 ppm, full stop, no exceptions. Freshwater fish are considerably less tolerant of nitrite than saltwater fish are — this isn't one where you get any real cushion.
• Nitrate: keep general community tanks under 20–40 ppm; more sensitive fish like discus are happier kept under 10–20 ppm long-term. It's not acutely toxic at aquarium levels, but chronic exposure wears on growth and immune function over time.
• Dissolved oxygen: worth a mention since it's easy to overlook — warmer water holds less of it, so a heavily stocked or heavily fed warmer tank needs real surface agitation to keep up.

Invertebrates (Shrimp & Snails)

This is the category where precision actually matters most, and where a mistake is least forgiving.

Shrimp grow by molting — periodically shedding their entire exoskeleton and growing a new, larger one underneath. Right after a molt, that new shell is soft and essentially uncalcified, and the shrimp spends the next day or two pulling calcium directly from the surrounding water to harden it back up. That's exactly why a GH crash is so much more dangerous around molting specifically than at any other time — a freshly molted shrimp with nothing to harden its shell with is defenseless and often doesn't survive. Copper is the other one that catches people off guard, and the biology behind it is almost ironic: crustaceans use a copper-based molecule called hemocyanin to carry oxygen in their blood, instead of the iron-based hemoglobin fish and humans rely on. A copper-based medication doesn't just poison shrimp generically — it interferes with the very metal their respiratory pigment depends on, which is part of why the safe margin for copper exposure is so much narrower for shrimp than it is for fish. Snails run into their own version of the calcium problem: their shells are calcium carbonate, so in soft, low-GH water the shell is thermodynamically pulled toward dissolving back into solution rather than staying intact — the same equilibrium problem a coral skeleton faces, just running in reverse.

• Caridina (Crystal Red, Bee shrimp): GH 4–6, KH 0–2, pH 6.2–6.8, temperature 68–75°F. They do not like heat.
• Neocaridina (Cherry shrimp and similar): GH 6–8, KH 3–6, pH 7.0–7.5, temperature 68–78°F. Noticeably more forgiving across the board.
• Snails (Mystery, Nerite): want moderate-to-high GH, roughly 6–12+, since they're pulling calcium straight from the water to maintain their shells. Run it too soft for too long and you'll see shell pitting and erosion.
• Ammonia and nitrite: 0 ppm — inverts are, if anything, even less forgiving here than fish.
• Copper: never treat a shrimp or snail tank with a copper-based medication, and be careful about what's already in a product before dosing a mixed tank.
• Nitrate: keep it low, ideally under 20 ppm — shrimp are noticeably more sensitive to nitrate spikes around molting than most fish are.