How I Finally Stopped Killing Fish and Started Understanding Water Chemistry

Water chemistry used to be the thing that made me want to throw my test kit across the room and give up on aquascaping entirely. I’d set up these tanks that looked amazing in my head – you know, lush plants, happy fish, the whole Instagram-worthy setup. Then within a week everything would go to hell. Plants turning to mush, fish gasping at the surface, water looking like chocolate milk. Fun times.

For the longest time I just blamed everything else. The plants must be defective. The substrate was probably garbage. Maybe I had bad karma or something, I don’t know. It took way too many dead fish and crashed tanks before I finally accepted that maybe, just maybe, I should actually understand what was happening in the water instead of just hoping for the best.

My wake-up call came when I agreed to set up a simple planted tank for this restaurant in Short North. Should’ve been easy money – 20 gallons, nothing fancy, just something that looked nice and didn’t require a lot of maintenance. I spent hours getting the hardscape just right, planted everything carefully, added some peaceful community fish. Looked perfect when I left.

Three days later I get this frantic phone call at like 7 AM. The manager is practically yelling about how the water looks like milk and there are dead fish floating around. Great. Just great. I threw on clothes and raced over there with my testing supplies, fully expecting to find some nightmare ammonia situation because they’d been overfeeding or something.

Turns out the brilliant staff had been topping off the tank with water from their commercial water softener system. You know, the kind designed to remove every trace of mineral from water for dishwashing. The water was so demineralized it was basically toxic to anything living in it. I spent the next six hours doing emergency water changes with bottled spring water I had to buy from three different grocery stores, googling “fish dying from soft water” on my phone between trips to the parking lot with buckets.

Not exactly my finest professional moment, but it definitely kicked off my obsession with actually understanding water chemistry instead of just winging it.

I’ve filled probably four notebooks with water test results over the years since then. Tried every additive and substrate combination you can imagine. And yeah, I’ve killed way more fish than I’m comfortable admitting while figuring this stuff out. But I’ve learned a ton, mostly through making every possible mistake, so maybe I can save you from some of the same disasters.

Let’s start with pH, because everyone tests for it but most people don’t really get what it means. It’s basically measuring how acidic or basic your water is, from 0 to 14, with 7 being neutral. Most freshwater fish do fine somewhere between 6.5 and 7.8, though there are definitely exceptions depending on what you’re keeping.

Here’s the thing though – and this took me way too long to figure out – stability matters way more than hitting some magic number you read about online. I’ve got cardinal tetras that have been thriving for two years in pH 7.5 water, which is technically higher than they supposedly prefer. But it’s been rock solid at 7.5 the whole time, so they’re perfectly happy.

The biggest mistake I see people make is chasing that perfect pH number like they’re trying to win some contest. They’ll dump pH-down and pH-up products in their tank multiple times a day, testing constantly, creating these wild swings that stress their fish way more than a stable pH that’s maybe not textbook perfect.

I watched this guy at my local fish store literally add pH-down to his tank three separate times in one afternoon. His fish were flashing and darting around like they were being electrocuted, and he couldn’t understand why. The constant pH fluctuations were driving them crazy, but he was so focused on getting to 6.8 that he didn’t notice his fish were having nervous breakdowns.

My approach now is pretty straightforward – I try to match fish to my water instead of fighting what comes out of my tap. Columbus water runs about 7.6 pH with moderate hardness, so I stock fish that are okay with those conditions. When I want to keep species that need softer, more acidic water, I use methods that create gradual, stable changes.

For a South American biotope tank I set up last year, I needed to get the pH down to around 6.0 for some wild-caught apistos. Instead of chemical warfare, I loaded the tank with oak leaves and catappa leaves, added some peat to the canister filter, and used an acidic substrate designed for blackwater setups. Took about two weeks for the pH to drop slowly and naturally, and it’s been stable ever since with basically no intervention from me.

The key word there is patience, which honestly is in pretty short supply in this hobby. Everyone wants results yesterday, including me sometimes.

Now the nitrogen cycle – this is absolutely fundamental stuff, but I’m constantly amazed by how many experienced fishkeepers still don’t really understand it. The basic version is: fish produce ammonia, which is highly toxic. Beneficial bacteria convert that ammonia to nitrite, which is also toxic. Different bacteria convert nitrite to nitrate, which is less immediately dangerous but still problematic if it builds up too much.

This is happening 24/7 in your tank, and if you don’t understand this process, you’re basically flying blind. My most embarrassing confession here is that I kept fish for years as a teenager and never really understood cycling. I thought it just meant running the filter for a few days before adding fish. The number of fish I killed through ignorance still makes me feel guilty when I think about it.

Testing for ammonia and nitrite is non-negotiable, especially in new setups. Both should read zero in a healthy, established tank. Any detectable ammonia or nitrite means you’ve got problems that need immediate attention. I test weekly even in my established tanks, not because I expect issues, but because catching problems early beats dealing with disasters later.

One thing to watch out for – test kits aren’t foolproof. I once spent three weeks trying to figure out why I kept getting weird ammonia readings in a tank that seemed perfectly fine. Turns out my API test kit had expired like six months earlier and was giving me false positives. Now I write the purchase date on every test kit with a Sharpie and replace them religiously, even if I haven’t used them much.

Nitrate is the end product of the nitrogen cycle, and managing it is where things get interesting. Fish can handle moderate nitrate levels – generally anything under 40ppm is considered safe. But plants actually need nitrate as fertilizer, so in heavily planted tanks you might struggle to maintain detectable levels.

In my main 40-gallon planted tank with a light fish load, I’ve gone months without water changes because the plants consume nitrate as fast as it’s produced. The readings stay at zero consistently. But in my fish-only tanks or lightly planted setups, I have to do regular water changes to keep nitrate from building up. I usually shoot for keeping nitrate somewhere between 5-20ppm in most planted tanks – enough to feed the plants without getting too high for sensitive fish.

One frustrating thing about nitrate testing – the API kit that everyone uses requires this ridiculous shaking protocol for the second bottle. You literally have to bang it against a hard surface for 30 seconds, which feels like forever when you’re actually doing it. Skip this step or don’t shake it enough, and you’ll get completely wrong readings. I’ve had people swear they had no nitrate problems when their actual levels were over 80ppm because they didn’t shake the reagent properly.

Water hardness is another parameter that doesn’t get enough attention. General Hardness (GH) measures dissolved minerals like calcium and magnesium. Carbonate Hardness (KH) measures carbonates and bicarbonates, which buffer against pH swings. Most test kits measure both in degrees (dGH/dKH) or parts per million.

The thing about hardness is that it’s directly connected to pH stability. I’ve watched people struggle for months trying to lower pH in tanks with high KH, not understanding that the buffering capacity of their water was working against them. It’s like trying to drive with the parking brake on – you might make some progress, but it’s going to be a fight the whole way.

I learned about hardness the hard way when I tried keeping some expensive wild discus in my regular Columbus tap water. I adjusted the pH to their preferred range, everything looked good on paper, but they never colored up properly and always seemed stressed. After consulting with a breeder who actually knew what he was doing, I tested my GH and discovered it was nearly five times higher than what discus experience in the wild.

Switching to RO water remineralized to the correct hardness transformed those fish within weeks. Their colors became incredible and they started breeding behavior almost immediately. Expensive lesson, but definitely worth learning.

Water changes are probably the most important tool for managing chemistry, and there’s still this ongoing debate about whether big weekly changes or small daily changes work better. Honestly, I’ve had success with both approaches depending on the specific tank. For most of my display tanks, I do 30-50% changes once a week. For breeding setups or tanks with very particular chemistry requirements, smaller changes every day or every other day keep things more stable.

Temperature matching is absolutely critical for water changes. I learned this lesson the hard way in my first apartment, which had this ancient hot water heater that would randomly change temperature. During one water change, I accidentally added water that was probably 10 degrees cooler than the tank. The temperature shock triggered bacterial infections that wiped out half my fish, including an angelfish pair I’d been trying to get to spawn for months.

Now I’m obsessive about temperature matching. I’ve got a digital thermometer that I use to test every bucket of change water before it goes in the tank. Takes an extra minute but it’s saved me from disasters multiple times.

Maybe the most underrated aspect of water chemistry management is just watching your tanks carefully. Test results and numbers matter, but nothing replaces spending time observing your fish and plants. I make it a point to really look at each tank for at least a few minutes every day – how the fish are behaving, whether the plants look healthy, if algae is showing up in new places.

You can often spot chemistry problems before they show up on test kits. Fish flashing against decorations might indicate pH swings. Increased breathing rates could mean ammonia stress. Plants developing holes or weird coloration might signal mineral deficiencies. After doing this for years, you start developing an instinct for when something’s off, even if you can’t immediately identify what it is.

After fifteen years of obsessing over parameters, making every possible mistake, and gradually figuring out what actually works, I’ve come to a surprisingly simple conclusion: stability beats perfection every single time. I’ve seen gorgeous, thriving tanks with parameters that shouldn’t work according to the textbooks. I’ve also seen perfectly optimized systems crash for mysterious reasons.

The tanks that succeed long-term are almost always the ones where parameters stay consistent, even if they’re not technically ideal. Water chemistry can feel overwhelming when you’re starting out – trust me, I remember feeling completely lost in all the abbreviations and conflicting advice. But really, it comes down to creating a stable environment where your fish and plants can do their thing. Get that figured out, and everything else becomes much more manageable.