What Water Parameters Should I Test for My Aquarium?

Why This Actually Matters (And Why I Care)

When I started keeping fish 8 years ago, I thought water testing was some nerdy extra step. My logic was: “If I can’t see anything wrong, it’s probably fine, right?”

Wrong. So wrong.

I lost an entire tank of tetras in 36 hours because my ammonia spiked to 2.0 ppm and I had no idea. The water looked fine. The fish looked fine on Monday. By Wednesday, half were dead. By Friday morning, all of them were gone.

That’s when I learned: your eyes lie to you. Fish can handle a lot, but once the water goes toxic, it goes fast. Really fast.

The Honest Answer: What You ACTUALLY Need to Test

Forget the 47-parameter test kits. Here’s what actually matters, broken down by priority:

Tier 1: Test These or Your Fish WILL Die

These three are non-negotiable. Period.

1. Ammonia (NH₃/NH₄⁺)

Target: 0 ppm. Not “close to zero.” Actually zero.

Ammonia is fish pee and poop broken down into toxic gas. Even 0.25 ppm burns gills like you’re breathing bleach. At 1.0 ppm, you’ve got maybe 48 hours before fish start dropping.

Test it: Daily for new tanks (first 4-6 weeks). Weekly for established tanks. Immediately if fish gasp at the surface or hang near the filter output.

2. Nitrite (NO₂⁻)

Target: 0 ppm. Same deal—zero or bust.

Nitrite is what ammonia turns into when your beneficial bacteria start doing their job. Problem? It’s still toxic. It basically suffocates fish from the inside by blocking oxygen in their blood.

Test it: Daily during cycling (weeks 2-6 usually). Weekly for established tanks. Anytime you add a bunch of new fish.

Fun fact nobody tells you: nitrite readings can spike AFTER ammonia drops to zero. That’s normal during cycling, but it freaks everyone out the first time.

3. Nitrate (NO₃⁻)

Target: Under 20 ppm (some fish can handle 40 ppm, but lower is always better)

Nitrate is the final form—what nitrite becomes when the bacteria finish processing it. It’s way less toxic than ammonia or nitrite, but it still causes problems long-term. High nitrate = stressed fish = sick fish = eventually dead fish.

Test it: Weekly. More often if you have a heavily stocked tank or you’re lazy about water changes (no judgment, we’ve all been there).

Tier 2: Test These to Keep Fish Happy (Not Just Alive)

4. pH

Target: Depends on your fish, but 6.5-7.5 works for 90% of community tanks

Here’s the thing about pH that nobody explains well: stability matters way more than the exact number.

I’ve seen tanks at pH 6.2 with thriving fish, and tanks at perfect 7.0 where everything died. Difference? The first guy’s pH stayed at 6.2 for months. The second guy’s pH swung from 6.8 to 7.4 every week because he kept messing with it.

Test it: Weekly for the first month, then monthly once it’s stable. Test again if you notice weird behavior (fish gasping, hiding, not eating).

What to do about it: Honestly? Unless your pH is below 6.0 or above 8.5, just leave it alone. Most fish adapt to your water better than they handle pH swings from you “fixing” it.

5. GH (General Hardness)

Target: 4-12 dGH for most community fish (some softwater species want 2-6 dGH)

GH measures dissolved minerals—mainly calcium and magnesium. Soft water has low GH, hard water has high GH.

Why it matters: Some fish evolved in soft rainforest streams (tetras, discus). Others evolved in hard mineral-rich lakes (livebearers, African cichlids). Put them in the wrong water and they get stressed.

Test it: Once when you set up the tank, then every few months. It doesn’t change much unless you’re actively trying to alter it.

6. KH (Carbonate Hardness / Alkalinity)

Target: 4-8 dKH (or 70-140 ppm)

KH is your pH stability buffer. High KH = stable pH. Low KH = pH swings like crazy.

I ignored KH for years until I kept having mysterious pH crashes that killed half my tank overnight. Turns out my KH was 1 dKH—so low that CO₂ from fish breathing was enough to tank my pH from 7.2 to 5.8 overnight.

Test it: Same as GH—once at setup, then every few months. Test more often if your pH randomly swings.

Tier 3: Advanced Stuff (You Probably Don’t Need This)

These parameters matter for specific setups, but most people never need to test them:

• Phosphate (PO₄³⁻): Only matters if you have algae problems or a heavily planted tank. Target: 0.5-2.0 ppm for planted tanks, as low as possible otherwise.
• Copper (Cu²⁺): Only test if treating disease with copper medication or if your tap water runs through old copper pipes.
• Chlorine/Chloramine: Your water conditioner handles this. If you’re using conditioner, you’re fine.
• TDS (Total Dissolved Solids): Only for shrimp keepers or people breeding sensitive fish.

What Tests Should YOU Buy?

Okay, so you’re sold on testing. What do you actually buy?

For Brand New Tanks (First 2 Months)

API Freshwater Master Test Kit — $25-35

Tests ammonia, nitrite, nitrate, pH (regular and high range). Lasts for 800+ tests. Pays for itself after 2-3 months vs. buying individual tests.

Yeah, the color matching is annoying. Yeah, you’ll second-guess yourself on nitrite readings. But it’s accurate enough and way cheaper than strips long-term.

For Established Tanks (After 6 Months)

Once your tank is stable, you can get lazy (in a good way):

• Nitrate test only: $8-12 for API Nitrate Test Kit. Test weekly.
• Maybe pH: If it’s been stable for 6+ months, test monthly or when something seems off.
• GH/KH: Test once a year unless you’re dosing minerals.

Ammonia and nitrite? You can stop testing these if:

1. Your tank has been running 6+ months
2. You do regular water changes
3. You haven’t added a ton of new fish recently
4. Nobody’s sick or dying

BUT—keep the ammonia test kit around. If fish start dying randomly, test ammonia FIRST.

How Often Should You Actually Test?

Here’s my real-world schedule (not the paranoid every-day schedule some guides recommend):

Red Flags That Mean “Test Your Water RIGHT NOW”

Even if you test on schedule, drop everything and test immediately if you see:

• Fish gasping at the surface (test ammonia first, then nitrite)
• Multiple fish hiding when they’re usually active (test pH and ammonia)
• Sudden deaths (2+ fish in 24 hours) → test EVERYTHING
• Fish hovering near filter output or bubbler (low oxygen, probably from ammonia/nitrite)
• Cloudy or smelly water (bacterial bloom from ammonia spike)
• After adding 3+ new fish at once (ammonia can spike from bioload increase)

Common Mistakes I See All The Time

1. “My water looks clear, so it must be fine”

Water can have 4.0 ppm ammonia and look like bottled spring water. Visual clarity means nothing. Test the water.

2. “I’ll test when I notice a problem”

By the time you “notice a problem,” your fish have been suffering for days. Ammonia at 0.5 ppm doesn’t show symptoms immediately—but it’s burning gills the whole time.

3. “Test strips are just as good”

They’re not. Strips are fine for established tanks checking nitrate/pH. But for ammonia during cycling? They’re off by 0.5-1.0 ppm regularly. That’s the difference between “safe” and “dead fish.”

4. “My pH is 7.8 but the guide says 7.0, so I’ll fix it”

Stop. Don’t mess with pH unless it’s extreme (below 6.0 or above 8.5). Most fish adapt to stable pH way better than they handle you dosing chemicals every week.

5. “I don’t need to test, I just do big water changes”

Water changes help, but they’re not magic. If your biofilter crashes, a 30% water change just drops your ammonia from “lethal” to “still really bad.” You need to know the actual numbers.

The Bottom Line (What I Wish Someone Told Me 8 Years Ago)

Here’s the honest truth:

In the first 2 months? Yes, you need to test religiously. Ammonia and nitrite during cycling aren’t optional—they’re life or death.

After 6 months with a stable tank? You can relax a bit. Weekly nitrate tests and occasional pH checks are usually enough.

When something goes wrong? Test everything. Every time. No exceptions.

I know testing feels like homework. I know it’s annoying to match colors on tiny vials. But here’s what I’ve learned after keeping 12 tanks over 8 years:

5 minutes of testing once a week prevents 5 hours of emergency treatments and hundreds of dollars in lost fish.