How Much Water Should I Change in My Aquarium? A Calm, Calculator-Based Way to Decide

Keeping fish and shrimp is supposed to be relaxing, but water changes can easily become the most stressful part of the hobby.

I used to “just change half” whenever the water looked a bit cloudy. Sometimes it was fine. Other times, especially in shrimp tanks, I would see stressed behaviour, awkward moults, or even losses a day or two later.

Over time I realised the problem wasn’t only how often I changed water, but how much – and that I was guessing both my real tank volume and my safe water-change percentage.

This article is how I now approach water changes more calmly, and how I use the Aquarium Volume & Water Change Calculator to stop guessing and start planning.

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Why “Just Change Half” Can Be Too Much for Some Tanks

“Just change 50% every week” is common advice. It works for many fish tanks if:

• Your tap water is similar to your tank water

• You keep hardy fish

• You don’t have sensitive species like Caridina or fragile Neocaridina lines

In shrimp tanks or very soft water setups, a sudden 50% change can feel like a mini reset:

• TDS (total dissolved solids) can swing sharply

• KH (carbonate hardness) can jump or crash

• Temperature can drop a few degrees if the refill water is cooler

Shrimp don’t usually collapse instantly. They often react in the next 24–48 hours: incomplete moults, hiding, colour fading, or just “not acting right”. That’s why many people don’t connect the issue back to a big, fast water change.

For me, the turning point was realising:

A smaller, consistent water change that keeps parameters stable is usually safer than a big, heroic water change after I’ve ignored the tank for weeks.

So instead of thinking in “half the tank”, I started thinking in percentages based on real water volume.

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Real Tank Volume vs What the Box Says

Most of us start with the number on the sticker:

• “60 cm tank – 60 litres”

• “90P – 160 litres”

• “2 ft tank – around 60 litres”

But that number assumes:

• The tank is filled all the way to the rim

• There’s no substrate

• There’s no rock, wood, or internal filter displacing water

In real life, especially in aquascapes and shrimp tanks:

• We leave some space at the top

• We use thick substrate

• We add hardscape, sometimes quite heavy

That means your “60 litre tank” might only hold 40–45 litres of actual water.

This matters a lot when you’re calculating:

• How many litres to change

• How much dechlorinator or remineraliser to dose

• How much any change will shock your animals

So the first step is to estimate what I call the real, usable water volume.

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How I Estimate Real Volume

I do this in three simple steps.

1. Measure the inner dimensions and water height

I measure inside the glass, not outside:

• Inner length (cm)

• Inner width (cm)

• Current water height from substrate surface up to the water line (cm)

Then I multiply:

Length × Width × Water Height ÷ 1000 = Approximate litres

Example:

• Length: 60 cm

• Width: 30 cm

• Water height: 32 cm

60 × 30 × 32 = 57,600 cm³
57,600 ÷ 1,000 = 57.6 L (if there was no hardscape or substrate volume counted)

This is my “box of water” estimate.

2. Think about what’s inside the box

Now I look at what’s taking up space:

• Deep substrate bed? (especially in natural / soil-based tanks)

• Big rocks or road-like stone lines

• Large pieces of driftwood

• Internal filters, sponge filters, or background filters

Together, these can easily displace 10–30% of the volume.

3. Apply a displacement estimate

Instead of overthinking it, I use simple bands:

• Light hardscape, shallow substrate → subtract ~10%

• Normal planted tank, decent hardscape → subtract ~15–20%

• Deep substrate, lots of stone/wood → subtract ~25–30%

If my “box of water” is 57.6 L and I estimate 20% displacement:

20% of 57.6 ≈ 11.5 L
57.6 – 11.5 ≈ 46 L real water

This is much closer to what I’m actually changing during maintenance.

The Aquarium Volume & Water Change Calculator basically helps with this thought process: you enter your dimensions and your estimated displacement, and it gives you a more realistic working volume.

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How I Decide My Water Change %

Once I know my real tank volume, the next question is:

“Okay, but should I change 10%, 20%, 30%… how much is safe for this tank?”

I look at three main things:

1. Stocking level – lightly vs heavily stocked

2. Type of animals – shrimp vs bigger fish

3. How different my tap water is – especially TDS, GH, KH

Lightly Stocked vs Heavily Stocked

• Lightly stocked tank
  A shrimp tank with lots of plants, low feeding, and gentle filtration usually runs quite “clean”.

  • I’m comfortable with 10–15% weekly or 20% every 2 weeks, as long as I’m consistent.

• Heavily stocked tank
  A tank with many fish, messy eaters, or heavy feeding produces more waste.

  • I lean towards 20–30% weekly.

  • If nitrates are climbing fast or there’s no live plants, I might go a bit higher – but I still avoid sudden 50% changes unless there’s an emergency.

Shrimp vs Bigger Fish

For shrimp, stability is everything:

• I prefer smaller, regular changes:

  • 10–15% once a week

  • Or 5–10% twice a week if I’m home and have time

• I will only go above 20% if:

  • I know my tap TDS and KH are very close to the tank, and

  • I match temperature and remineralisation carefully

For bigger fish (hardy community fish, for example):

• They usually tolerate 20–30% quite easily

• They may even benefit from slightly larger changes if nitrates tend to accumulate

I’ve written more about keeping KH steady and stable in Keeping Your KH Steady – that article goes deeper into why sudden KH swings are a big stressor for shrimp and fish alike.

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Example: 60P Shrimp Tank Calculation

Let’s walk through a real example using a typical 60P shrimp tank.

Tank setup:

• Standard 60P (60 × 30 × 36 cm)

• Water level: about 4 cm below the rim → water height ≈ 32 cm

• Deep soil + sand cap

• A few decent-sized rocks and driftwood

• Lightly stocked Neocaridina with plants

Step 1 – Box volume

Using the inner dimensions:

• Length: 60 cm

• Width: 30 cm

• Water height: 32 cm

60 × 30 × 32 = 57,600 cm³
57,600 ÷ 1,000 = 57.6 L

Step 2 – Estimate displacement

Deep substrate and some hardscape? I’ll assume 20% displacement.

20% of 57.6 ≈ 11.5 L
57.6 – 11.5 ≈ 46 L real water volume

So I’ll treat this tank as roughly 45–46 litres of water, not “60 litres”.

Step 3 – Choose a calm water change %

Because it’s a shrimp tank that’s lightly stocked, I choose 15% weekly as my standard.

15% of 46 L ≈ 6.9 L

I round that to about 7 litres per water change.

That’s it. My “maintenance instruction” for myself becomes:

“For this 60P shrimp tank, change ~7 L once a week, with matched temperature and TDS.”

If water tests and shrimp behaviour look good over time, I keep that routine. If I see slow nitrate creep or a little film on the surface, I might nudge it to 20%:

20% of 46 L ≈ 9.2 L → about 9 litres

Still controlled, and still far from blindly swapping “half the tank”.

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Using the Aquarium Volume & Water Change Calculator

To make this simpler, I now use the Aquarium Volume & Water Change Calculator instead of doing mental math every time.

Here’s how I use it step-by-step:

1. Enter the tank dimensions

   • Inner length, width, and your usual water height (not the full tank height if you leave a gap at the top).

2. Set an estimated displacement %

   • Light substrate, minimal scape → around 10%

   • Normal planted scape → 15–20%

   • Heavy rock/wood, deep soil → 25–30%

3. Choose your water change %

   • For shrimp / lightly stocked: start with 10–15%

   • For heavier fish load: 20–30%, if your tap water is similar

4. Read your real working volume

   • The calculator will show your estimated real water volume, not the marketing number on the box.

5. Note the litres to change

   • It then gives you how many litres to remove and refill for your chosen percentage.

   • I like to write this number down in my water log so I don’t have to recalculate every weekend.

You can open the tool here:
Aquarium Volume & Water Change Calculator (toolkit page)

This article is the “why”; the calculator is the “how much” button I press when my brain is tired.

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Small Reminders Before You Change Water

Even with perfect calculations, the details still matter. Before any water change, I quietly check:

• Temperature
  Try to keep the new water within about 1–2°C of the tank. A big cold shock is stressful even if the percentage is small.

• TDS and hardness
  Especially for shrimp, I try to keep TDS, GH, and KH as close as possible between old and new water.

  • If you’re remineralising RO or very soft water, measure both.

  • I talk more about KH in Keeping Your KH Steady and about filter roles in Do We Really Need a Filter in a Natural Tank?

• Chlorine/chloramine
  Always use a reliable dechlorinator if you’re using tap water, even for small changes.

• Pouring speed
  I refill gently:

  • Use a jug, drip, or pour onto a plate to avoid blasting substrate and shrimp.

  • Fast refills stir up detritus and stress the animals.

• Watch after, not just during
  I observe the tank for a while after the water change and again later that day:

  • Are shrimp grazing normally?

  • Any fish gasping, hiding, or acting unusual?

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Connecting Volume, Water Changes, and Long-Term Stability

For me, the big shift was moving from guessing to measured routines:

• Knowing the real volume, not the marketing volume

• Choosing water change percentages based on animals and stocking, not random numbers

• Using tools like the Aquarium Volume & Water Change Calculator to make decisions calmly

Over months, this approach has made my tanks feel more predictable:

• Fewer surprises after water changes

• Less anxiety about “Did I change too much?”

• A routine that feels gentle for shrimp and sustainable for me

If you’re often unsure whether you’re changing too much or too little, start with:

1. Measure your real tank volume

2. Use the calculator to pick a calm water change %

3. Observe your animals and adjust slowly over time

From there, you can deepen the journey with related reads like:

• Keeping Your KH Steady – how I keep buffering stable

• Do We Really Need a Filter in a Natural Tank? – how filtration, plants, and substrate share the workload

And whenever you’re in doubt, you don’t have to guess. You can always go back to the calculator, take a breath, and let the numbers guide you.