What to do if your water is too acid or too alkaline

Why pH matters - and when it doesn't

pH is a measure of how acidic or alkaline your water is, on a scale of 0-14. Pure neutral water sits at 7.0. Most freshwater fish come from rivers and lakes that sit somewhere between 6.0 and 8.0, and many species have been tank-bred for so many generations that they tolerate a wider range than their wild counterparts ever did.

That said, pH does matter for a few concrete reasons. At very low pH (below 6.0), beneficial bacteria in your filter slow down significantly, which affects the nitrogen cycle and lets ammonia and nitrite creep up. At very high pH (above 8.5), ammonia becomes dramatically more toxic - the same ammonia reading that's borderline safe at pH 7.0 becomes genuinely dangerous at pH 8.5. And rapid swings - anything more than about 0.3 units in 24 hours - stress fish enough to suppress their immune systems, making them vulnerable to disease.

What most guides skip over: the number on your test kit is not the whole story. KH (carbonate hardness) is what actually stabilises pH. Low KH means low buffering capacity, which means pH can swing wildly, especially overnight as CO₂ levels change. If your pH keeps moving around, check your KH before doing anything else. A KH below 3-4 dKH is asking for trouble.

Understanding what's driving your pH

Before you try to fix anything, work out why your pH is where it is. The same reading can have completely different causes, and the fix depends on the cause.

What lowers pH

• Soft tap water with low KH - common in areas with granite geology or treated municipal water. Low buffering means CO₂ from fish respiration and bacterial activity drives pH down, especially at night.
• Driftwood and botanicals - release tannins and humic acids over time. This is intentional in a blackwater setup, a problem if you didn't plan for it.
• Leaf litter - Indian almond leaves, catappa bark, and similar botanicals all pull pH downward.
• Heavy organic waste / high nitrates - decomposing waste produces CO₂ and organic acids. Tanks that are overstocked or undercleaned tend to drift acidic over time. Check out how often to do water changes if yours are irregular.
• Peat substrate or peat in the filter - deliberate, but worth knowing if you inherited a tank.

What raises pH

• Hard tap water with high KH - common in limestone geology areas. Your tap might come out at pH 7.8-8.2 with high GH and KH to match.
• Crushed coral, coral sand, or limestone decor - these dissolve slowly and continuously add carbonates, raising both KH and pH. Fine in a cichlid tank, a problem if you're trying to keep soft-water tetras.
• Calcareous rocks - limestone, tufa, and Texas holey rock all leach carbonates. Tap rocks you find outside against a drop of vinegar; if they fizz, they'll raise your pH.
• Very low CO₂ levels - in heavily planted tanks under bright light, plants consume CO₂ rapidly during the day, and pH can climb noticeably through the afternoon. This is normal and usually harmless if KH is adequate.

Water too acidic: how to raise pH safely

If your pH is consistently below 6.5 and you're keeping fish that prefer neutral-to-alkaline conditions - livebearers, most African cichlids, goldfish - you need to raise it. If you're keeping wild-caught Amazonian species intentionally in soft, acidic water, low pH may be exactly right for them.

Increase KH first

This is the correct first step almost every time. Low pH in a tank with adequate KH is unusual. Low pH in a tank with KH under 3 dKH is completely normal and expected. Raising KH gives your water buffering capacity, which stabilises pH and stops it swinging.

Add a small bag of crushed coral or crushed limestone to your filter. These dissolve slowly in acidic conditions and stop dissolving once KH and pH rise enough - a natural self-regulating mechanism. Start with a couple of tablespoons per 100 litres, test after 48-72 hours, and adjust from there. This is slower than adding chemicals, which is exactly why it's better.

Water changes with harder water

If your tap water has higher KH than your tank, regular water changes will gradually bring parameters closer to tap values. This is often sufficient for mild cases. The key is consistency - see how often to do water changes for guidance on frequency.

What not to do

Don't add liquid pH-raising products (sodium bicarbonate, commercial alkalinity boosters) without first understanding your KH. They'll raise pH temporarily, but without buffering capacity the pH will crash back - sometimes even lower than before, which is worse than a stable low pH.

Water too alkaline: how to lower pH safely

High pH is most common in hard-water areas where tap water arrives at 7.8-8.4 with high KH. It's also common in tanks decorated with calcareous rock or coral sand. Before doing anything, check whether the fish you're keeping actually care. Mollies, platies, swordtails, and most African Rift Lake cichlids actively prefer hard, alkaline water. Fighting your parameters to keep them acidic is unnecessary work.

Driftwood, peat, and botanicals

Adding driftwood is the most common approach, and it does work - slowly. Larger pieces of spider wood, mopani, or bogwood will gradually release tannins. Peat in a filter bag works faster. Indian almond leaves and catappa bark are useful for small tanks or breeding setups. None of these will dramatically shift pH in hard water with high KH; high carbonate hardness buffers against pH changes strongly. You'll typically see a drop of 0.2-0.5 units, not 1.5 units.

Diluting with RO or distilled water

If your tap is genuinely very hard (KH above 10 dKH, GH above 20 dGH), the most effective approach is to mix your tap water with RO or distilled water before it goes in the tank. A 50/50 mix roughly halves hardness and brings pH down meaningfully. You'll need to remineralise RO water with a product designed for this - pure RO with zero buffering capacity is unstable and not safe to use alone.

What not to do

Liquid pH-Down products (usually phosphoric acid) will lower pH, but they don't reduce KH. The carbonates are still there, and they'll neutralise the acid and push pH back up within hours unless you keep dosing. This creates instability that's worse for fish than the original high pH. The only exception is CO₂ injection in planted tanks, which lowers pH in a controlled, reversible way.

The best long-term solution: match fish to your water

This is the advice that saves the most time and frustration, and it's the one most beginners don't hear early enough. Your tap water has a natural chemistry, and working against it constantly is expensive, time-consuming, and introduces instability. Working with it is much easier.

If you have hard, alkaline tap water (pH 7.8-8.2, high KH), lean into it. Livebearers like guppies, platies, mollies, and swordtails thrive in it. So do most Central American cichlids and shell-dwelling Tanganyikan cichlids. Check the best fish for beginners for options that suit a range of tap water types.

If you have soft, slightly acidic tap water (pH 6.5-7.0, low KH), you're set up well for South American species - most tetras, corydoras, dwarf cichlids like Apistogramma species, and many rasboras from Southeast Asia. Add a small crushed coral buffer to prevent pH crashes, and enjoy the fact that these fish are often more colourful in soft water.

How to test and track pH reliably

Liquid reagent test kits (API, Salifert, Tetra) are more accurate than strips for pH. That said, strips are fine for spotting trends - if a strip shows pH 6.2 when you were expecting 7.0, trust it enough to do a liquid test to confirm.

Test pH and KH together. A single pH reading without KH context tells you less than you think. If pH is low but KH is 5 dKH, your buffer is probably adequate. If pH is low and KH is 1 dKH, your tank is unstable and a crash is possible.

Test at a consistent time of day. In planted tanks, pH can shift 0.5-0.8 units between early morning (when CO₂ has built up overnight) and late afternoon (after plants have absorbed CO₂ all day). Neither reading is wrong - but comparing a morning reading one week to an afternoon reading the next will look like your pH is swinging when it isn't. For the full picture on which parameters matter and why, see our aquarium water parameters guide.