Quick Answer: Three numbers matter. Ammonia must be 0.00 mg/L. Nitrite must be 0.00 mg/L. Nitrate should be under 20 mg/L for freshwater, under 10 mg/L for saltwater. If any of these are off, your fish are stressed — even if they look fine. I've tested water parameters on probably 500+ tanks. The customers who track these three numbers weekly? They never have unexpected crashes. The customers who only test when something looks wrong? They call me when it's already too late.
0.00. 0.00. Under 20.
Those are the three numbers you need to remember.
Ammonia = 0.00 mg/L. Nitrite = 0.00 mg/L. Nitrate = under 20 mg/L.
That's it. That's the whole water chemistry of a healthy aquarium.
Everything else — pH, GH, KH, phosphate — is secondary. Those three numbers are the foundation. If they're off, nothing else matters.
Let me break down each one.
Ammonia (NH₃): The Silent Killer
What it is: Ammonia is fish waste. It's what fish excrete. It's also what decaying food and plant matter produce.
What it does: Ammonia is toxic to fish. Even at low levels, it causes stress. At 1.0 mg/L, it causes gill damage. At 2.0 mg/L, it starts killing fish.
What it should be: 0.00 mg/L. Always.
If ammonia is detectable? Something is wrong. Either your filter isn't working, you're overfeeding, or your tank isn't cycled.
What causes high ammonia:
- Overfeeding
- Dead fish in the tank
- Filter media cleaned with tap water
- New tank not cycled
- Too many fish
What to do about it:
- Do a 25% water change
- Dose with bottled bacteria
- Reduce feeding
- Don't add any more fish
Nitrite (NO₂⁻): The Middle Step
What it is: Nitrite is the middle step in the nitrogen cycle. It's what ammonia turns into, before it turns into nitrate.
What it does: Nitrite is also toxic. It prevents fish from carrying oxygen. At 1.0 mg/L, fish start gasping. At 5.0 mg/L, fish die.
What it should be: 0.00 mg/L. Always.
If nitrite is detectable? Your tank is in the middle of the cycle. The Nitrosomonas bacteria are working, but the Nitrobacter bacteria haven't caught up yet.
What causes high nitrite:
- New tank cycling
- Filter media cleaned too thoroughly
- Adding too many fish at once
What to do about it:
- Do a 20-30% water change
- Add Seachem Prime (it detoxifies nitrite)
- Wait for the Nitrobacter colony to grow
Nitrate (NO₃⁻): The Final Product
What it is: Nitrate is the final product of the nitrogen cycle. It's what nitrite turns into.
What it does: Nitrate is much less toxic than ammonia or nitrite. But at high levels, it still causes stress. It also fuels algae growth.
What it should be:
- Freshwater: under 20 mg/L
- Saltwater: under 10 mg/L
- Reef tank: under 5 mg/L
What causes high nitrate:
- Not enough water changes
- Overfeeding
- Too many fish
- Live plants not consuming enough
What to do about it:
- Do a 25% water change
- Reduce feeding
- Add live plants (they consume nitrate)
- Upgrade to a bigger filter
The 0.00 Myth
Here's something I see all the time.
Customers say: "My ammonia is 0.00. My nitrite is 0.00. So my tank is perfect, right?"
Not necessarily.
If nitrate is at 50 mg/L? That's a problem. It's not killing fish immediately, but it's causing long-term stress. It's fueling algae growth. It's making the tank dirty.
Ammonia at 0.00 and nitrite at 0.00 just means the cycle is working. It doesn't mean the water is clean.
You need all three numbers in the right range. Not just two.
The Test Kit Problem
Here's the other thing I see all the time.
Customers have a test kit. They test once. Ammonia is 0.00. They never test again.
That's not enough.
You need to test weekly. Write down the numbers. Track the trend.
If ammonia starts at 0.00 and starts creeping up? That's an early warning. You catch it before it becomes a problem.
If you only test when something looks wrong? It's already too late.
What I'm Not Going to Tell You
I can tell you what the three numbers should be. I can tell you what causes them to go up. I can tell you how to fix it.
But I'm not going to tell you how to set up a planted tank. I'm not going to tell you which test kit is "the best." I'm not going to tell you how to dose your reef system.
That's not my department. I track these three numbers. I tell you what they should be. That's it.
The Three Numbers
0.00. 0.00. Under 20.
That's the whole guide.
If you remember those three numbers? You'll catch problems early. You won't have unexpected crashes. Your fish will be healthy.
If you don't remember them? You'll be calling me when something goes wrong.
What We Tell Customers at Ruibit
When we deliver a custom aquarium at Ruibit, we give every customer a water test log.
We tell them: test ammonia, nitrite, and nitrate every week. Write down the numbers.
We also tell them: if any number goes outside the range? Call us. Don't wait.
That's what we've learned from 500+ tank consultations. The customers who track these three numbers weekly? They have stable tanks. The customers who only test when something looks wrong? They call us when it's already a crisis.
The Three Numbers, Again
0.00. 0.00. Under 20.
That's it. That's the whole guide.
Write it down. Put it on your fridge.
