Freshwater Aquarium Water Parameters
Understanding your aquarium's water chemistry is one of the most important parts of successful fishkeeping. This guide focuses on tropical freshwater and coldwater aquariums. If you keep a marine aquarium, many of the principles are similar, but marine systems require additional water tests such as salinity, calcium and magnesium. You can read our dedicated Marine Water Parameters Guide here. Three of the most commonly measured water parameters are pH, GH (General Hardness) and KH (Carbonate Hardness). Although these terms can seem confusing at first, they're actually quite straightforward once you know what they measure. At Birchwood Aquatic Centre, we regularly help customers understand their water chemistry through free in-store water testing and practical advice. This guide explains what each parameter means, why it matters, and when (if ever) you should make adjustments.
What Are Aquarium Water Parameters?
Water parameters describe the chemical characteristics of your aquarium water.
The three most important are:
- pH – measures how acidic or alkaline the water is.
- GH (General Hardness) – measures the amount of dissolved calcium and magnesium.
- KH (Carbonate Hardness) – measures the water’s buffering capacity, helping to keep pH stable.
These values work together, so changing one can often influence another. Rather than chasing perfect numbers, the goal is to provide stable conditions that suit the fish you keep.
Stability Is More Important Than Chasing Numbers
One of the biggest mistakes beginners make is trying to constantly alter their pH using chemicals.
Fish cope far better with a stable pH than one that changes every few days. Unless you’re keeping specialist species, it’s usually better to choose fish that suit your local tap water rather than trying to force your water to match the fish.
Understanding pH
The pH scale runs from 0 to 14.
- Below 7 = Acidic
- 7 = Neutral
- Above 7 = Alkaline
Different fish have evolved in different water conditions.
For example:
- Amazonian fish such as Cardinal Tetras, Discus and many dwarf cichlids naturally prefer softer, slightly acidic water.
- Livebearers including Guppies, Mollies and Platies generally prefer harder, more alkaline water.
- African Rift Lake cichlids require alkaline conditions.
- Marine aquariums naturally run at a higher pH than freshwater aquariums.
However, most captive-bred tropical fish sold in the UK adapt well to a pH between 6.5 and 8.0, provided the water remains stable.
Understanding GH (General Hardness)
GH measures the concentration of dissolved minerals, primarily calcium and magnesium.
These minerals are important for:
- Healthy fish development
- Osmoregulation
- Invertebrates such as shrimp and snails
- Strong plant growth
General guide:
GH
Water Type
0–6 dGH
Soft water
6–12 dGH
Moderately hard
12+ dGH
Hard water
Many areas of southern England, including much of Kent, naturally have hard water, making them ideal for species such as:
- Guppies
- Mollies
- Platies
- Swordtails
- African Cichlids
Fish such as Discus, many wild-caught tetras and certain dwarf cichlids generally prefer softer water.
If softer water is required, RO (Reverse Osmosis) water can be mixed with tap water to achieve the desired hardness in a controlled and predictable way.
Understanding KH (Carbonate Hardness)
KH measures the concentration of carbonates and bicarbonates that help buffer the water against sudden pH changes.
Think of KH as your aquarium’s pH stabiliser.
Higher KH means the pH is more resistant to change.
Lower KH means the pH can change more easily as acids naturally build up within the aquarium.
Typical freshwater guide:
KH | Buffering Capacity |
0–3 dKH | Low |
4–8 dKH | Moderate |
9+ dKH | High |
Low KH doesn’t automatically mean there’s a problem, but it does mean you’ll need to monitor pH more closely.
How pH, GH and KH Work Together
These three values are closely linked.
- GH determines the mineral content of the water.
- KH helps stabilise pH.
- pH measures the acidity or alkalinity.
Although GH and KH often increase together, they measure different things and one can be high while the other remains relatively low.
Understanding all three gives you a much clearer picture of your aquarium’s water chemistry.
Test Before You Treat
Before adding any products to alter your water chemistry, always test your water first.
Many aquarium problems are caused not by the actual water parameters, but by making sudden changes.
Only adjust pH, GH or KH when there’s a genuine need, and always make changes gradually.
In many cases, leaving stable water alone is the best approach.
Testing Your Aquarium Water
Regular testing helps identify problems before they affect your fish.
We recommend testing:
- Weekly in established aquariums.
- Before adding new fish.
- After large water changes.
- If fish show signs of illness or unusual behaviour.
- Whenever algae or water quality issues appear.
Liquid test kits generally provide greater accuracy than test strips, especially when measuring pH and hardness.
If you’re unsure about your results, bring a sample into Birchwood Aquatic Centre and we’ll happily test it for you and explain what it means.
UK Tap Water
One of the advantages of fishkeeping in the UK is that your local water supply is generally very consistent.
However, hardness varies considerably depending on where you live.
Some areas have naturally soft water, while much of southern England has hard, alkaline water.
Rather than trying to dramatically alter your tap water, it’s often easier—and healthier—to choose fish that naturally suit your local water conditions.
Frequently Asked Questions
What is the ideal pH for a tropical aquarium?
Most community tropical fish do well between pH 6.5 and 8.0, provided the water remains stable. Different species have different preferences, so always research the fish you intend to keep.
Should I try to change my aquarium pH?
Usually not.
Stable water is almost always better than constantly adjusting pH with chemicals. Unless you’re keeping specialist species, it’s generally better to keep fish suited to your local water supply.
What's the difference between GH and KH?
GH measures the amount of dissolved calcium and magnesium in the water.
KH measures the carbonates and bicarbonates that help prevent sudden changes in pH.
Although they’re both described as “hardness,” they measure completely different properties.
Why does my pH keep changing?
The most common cause is low KH. With little buffering capacity, natural acids produced by fish waste and the biological filter can gradually lower the pH over time.
Is hard tap water bad for tropical fish?
Not necessarily.
Many of today’s captive-bred tropical fish adapt very well to hard UK tap water. In fact, species such as Guppies, Mollies and Platies actually prefer it.
When should I use RO water?
RO (Reverse Osmosis) water is useful when keeping fish that require softer water than your tap supply provides, or when preparing water for marine aquariums. RO water should normally be remineralised or mixed with tap water before use.
How often should I test my aquarium water?
Weekly testing is recommended for most aquariums, with additional testing after adding fish, performing large water changes or if your fish show signs of stress or illness.
Can I use bottled pH adjusters?
They can be useful in certain situations, but they’re often overused. Continually chasing a specific pH can create unstable water conditions. In most cases, maintaining stable water is more important than achieving an exact number.
Need Help Understanding Your Water?
At Birchwood Aquatic Centre, we offer free in-store water testing and friendly advice to help you understand your aquarium’s water chemistry.
Whether you’re setting up your first tropical aquarium, caring for a marine reef tank or maintaining a healthy pond, we’re here to help you keep your water conditions stable and your fish thriving.
Visit us in Swanley, Kent, or bring along a sample of your aquarium water and we’ll explain exactly what your results mean and recommend the best course of action.