Fish Tank Guide

7 Chemicals Your Fish Tank Actually Needs

5 min read · Updated August 30, 2026 · By Mark Whitfield

Setting up a freshwater fish tank in the UK means dealing with tap water that contains chlorine, chloramines, and heavy metals added by your water company. Without the right treatments, these chemicals damage fish gills and can kill them within hours. Here are the 7 chemicals and water treatments your aquarium genuinely needs, the ones you can skip, and which products work best for UK setups.

1. Water Conditioner (Non-Negotiable)

This is the single most important bottle on your shelf. UK tap water contains chlorine and chloramines to keep it safe for human drinking, but these same chemicals destroy the beneficial bacteria in your tank and burn fish gills. A water conditioner neutralises them in seconds. You need it for every water change, every top-up, every time new tap water enters the tank.

Seachem Prime is the go-to choice for most UK aquarists. It handles chlorine, chloramines, ammonia, nitrite, and heavy metals in one dose. A 100ml bottle treats roughly 1,000 litres of water, which covers months of water changes for a standard tank. For larger tanks or tighter budgets, API Aqua Essential (473ml) offers similar protection at a lower cost per litre.

2. Bacteria Starter (Essential for New Tanks)

When you set up a new aquarium, it needs to "cycle", a process where beneficial bacteria colonise the filter and break down fish waste into less harmful substances. Without these bacteria, ammonia builds up fast and kills fish. A bacteria starter seeds your tank with the right microorganisms so the cycle completes in 1-2 weeks instead of the natural 4-6 weeks.

Tetra SafeStart (250ml) contains live nitrifying bacteria and works when added directly to a tank with fish. It is the most commonly recommended starter in UK fish shops. Dose it on day one, keep the filter running, and test daily. The bacteria establish themselves within about a week if conditions are right.

3. Test Kit (Your Early Warning System)

You cannot see ammonia, nitrite, or nitrate. A test kit is the only way to know whether your tank is cycling properly, whether your water is safe, and whether something is going wrong before your fish show symptoms. During the first 6-8 weeks, test daily. After the cycle stabilises, test every 1-2 weeks.

API Master Test Kit is the gold standard. It tests ammonia, nitrite, nitrate, and pH with liquid reagents that give clear colour results. It runs about 800 tests, making it far cheaper per test than strips. If you want something faster for quick checks, API 5-in-1 Test Strips give results in 60 seconds, though they are less precise for ammonia and nitrite.

4. Stress Coat (When Fish Are Hurt or Stressed)

Fish have a natural slime coat that protects them from infections and parasites. Handling, netting, fighting, or rough water conditions can damage this layer. A stress coat product contains aloe vera compounds that help repair the slime coat and reduce stress hormones.

API Stress Coat (473ml) doubles as a mild water conditioner and a slime coat restorer. It is particularly useful when adding new fish, after tank maintenance that involves netting, or when you notice fish with cloudy patches or frayed fins. Not essential for day-to-day use, but worth keeping in the cupboard for emergencies.

5. pH Adjuster (Only If Your Water Is Way Off)

Most freshwater fish adapt to a pH range of 6.5 to 7.5. UK tap water typically falls within this range, so a pH adjuster is rarely needed. However, if your test kit consistently shows a pH below 6.0 or above 8.0, and you are keeping fish that need a specific range (like African cichlids that need 7.8-8.6), you may need to raise or lower it.

Use a pH adjuster sparingly and only after testing confirms the problem. Rapid pH swings are more dangerous than a slightly off reading. Change the pH gradually over several days, not all at once.

6. Anti-Parasite Treatment (Emergency Use)

White spot (Ichthyophthirius), velvet, and internal worms are the most common freshwater fish parasites in the UK. An anti-parasite treatment is not part of routine care, but you should have one on hand before you need it. By the time you spot symptoms like white spots, scratching against decorations, or weight loss despite eating, the parasite is already established.

Treatments containing formaldehyde and malachite green are the standard approach for white spot. Follow the dosage exactly and remove any activated carbon from your filter during treatment, as carbon absorbs the medication before it can work. Increase aeration too, as many anti-parasite chemicals reduce oxygen availability.

7. Algaecide (Use Sparingly)

Green water, brown algae on glass, and black beard algae on decorations are common in new or poorly lit tanks. An algaecide kills algae on contact, but it treats the symptom, not the cause. Algae grows because of excess light, excess nutrients, or an imbalance between the two.

Before reaching for chemicals, try reducing your lighting period to 6-8 hours per day, doing a 25% water change, and cleaning the glass. If algae persists after a week of those steps, a measured dose of algaecide can clear it. Overdosing algaecide kills beneficial bacteria and can crash your tank's cycle, so use the minimum effective amount.

Quick Shopping List

Day one essentials (budget roughly £25-35):

Seachem Prime 100ml - water conditioner
Tetra SafeStart 250ml - bacteria starter
API Master Test Kit - ammonia, nitrite, nitrate, pH

Keep in the cupboard (add as needed):

API Stress Coat 473ml - slime coat repair
Anti-parasite treatment (e.g. API General Cure or similar)
Algaecide (only if algae won't shift after manual cleaning)
pH adjuster (only if your water tests consistently outside 6.0-8.0)

Frequently Asked Questions

What chemicals do I need for a new freshwater fish tank?

At minimum you need a water conditioner to neutralise chlorine and chloramines from UK tap water, a bacteria starter to kickstart the nitrogen cycle, and a test kit to monitor ammonia, nitrite, and pH. Those three cover the essentials. As your tank matures, you may add a stress coat for fish health and an anti-parasite treatment for emergencies.

Do I need all 7 chemicals at once?

No. Water conditioner is non-negotiable from day one. Bacteria starter and a test kit are essential for cycling a new tank. The remaining four (stress coat, pH adjuster, anti-parasite treatment, and algaecide) are situational. You only need them when specific problems arise, not as part of a routine setup.

How do I cycle a new aquarium without chemicals?

You can cycle a tank naturally by adding a source of ammonia and waiting 4-6 weeks for beneficial bacteria to colonise. However, a bacteria starter like Tetra SafeStart speeds up the process to roughly 1-2 weeks. Either way, you still need a water conditioner to treat tap water and a test kit to track the cycle.

Can I use bottled water instead of tap water and chemicals?

Bottled water is not recommended for fish tanks. It often lacks the minerals fish need and may contain inconsistent pH levels. UK tap water treated with a conditioner is the most reliable and cost-effective option. The conditioner costs pence per litre and removes chlorine, chloramines, and heavy metals in seconds.

How often should I test my fish tank water?

Test weekly for the first 6-8 weeks while the tank cycles, then every 2-4 weeks once stable. During cycling, you are watching for ammonia and nitrite to spike and then drop to zero. After the cycle completes, a quick weekly test catches problems before they stress your fish.

Final Verdict

Freshwater fish tanks need a small, focused set of chemicals, not a shelf full of products. Start with a water conditioner, a bacteria starter, and a test kit. Those three handle 90% of what your tank needs. Keep a stress coat in the cupboard for emergencies, and only reach for pH adjusters, anti-parasite treatments, or algaecides when a specific problem demands it. The best aquarium chemistry is invisible, and your fish never know it is there.

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Written by Mark Whitfield

Mark writes about pet products and aquariums for WhoPets. He has kept freshwater aquariums for over 8 years and focuses on practical, honest product advice for UK pet owners.