You know what’s funny? When I first got into planted aquariums for my classroom, I thought lighting was going to be the easy part. I mean, how hard could it be, right? You buy a light, you put it over the tank, plants grow. Done. Man, was I wrong about that one.
I remember standing in the aquarium store, completely overwhelmed, staring at this wall of different light fixtures while some teenager with gauged ears tried to explain something called “PAR values” to me. All I wanted was to make my classroom tank look decent and keep some plants alive. The kid kept mentioning this watts per gallon rule – apparently if you wanted low light plants, you needed 1-2 watts per gallon, medium light was 2-3 watts, high light was 3+ watts. Simple enough, I thought. Got myself a basic fluorescent setup that hit about 2 watts per gallon and figured I was golden.
For my first setup, honestly, it worked fine. I had some basic plants – java fern, anubias, maybe some crypts – and they did okay. The kids could see the fish, the plants stayed green, nobody died. Mission accomplished from a seventh-grade science teacher perspective. But then I started getting more ambitious (which, let’s be honest, is my pattern with pretty much everything I get interested in). I wanted carpeting plants, I wanted red plants, I wanted one of those amazing aquascapes you see online that look like underwater forests.
That’s when the watts per gallon thing completely fell apart on me.
I’ll never forget my first attempt at growing dwarf baby tears. Bought a little pot of Hemianthus callitrichoides from the local fish store, planted it carefully in my 20-gallon home tank, and waited for it to carpet. And waited. And waited some more. The stuff barely grew, what little growth it had was leggy and pale, and eventually it just melted away entirely. I was following the watts per gallon rule perfectly – had exactly 2.5 watts per gallon – so what was going wrong?
Turns out, and this took me way too long to figure out, the whole watts per gallon thing is basically useless with modern lighting. It was created back when everyone used basic fluorescent tubes, and even then it was pretty rough. But with LEDs, compact fluorescents, metal halides… watts just don’t tell you anything meaningful about how much usable light your plants are actually getting. It’s like trying to figure out how bright a room is by looking at how much electricity the light bulb uses. Doesn’t work that way.
What I should have been paying attention to was something called PAR – photosynthetically active radiation. That’s the actual amount of light energy that plants can use for photosynthesis, measured in micromoles per square meter per second. Sounds complicated, but it’s basically just “how much plant-usable light is hitting the substrate.” Low light plants need about 15-30 PAR, medium light wants 30-50, and high light plants need 50+ PAR to really thrive. Those numbers actually mean something, unlike watts per gallon.
But here’s the thing – and this is where it gets frustrating for us teachers on tight budgets – PAR meters cost like $200-300. I wasn’t about to drop that kind of money just to measure my aquarium lights. Luckily, I discovered that a lot of the aquascaping forums have people who’ve already measured PAR values for common light fixtures at different distances and tank depths. You can usually find the information you need without buying your own meter, though you’ve got to do some digging.
The spectrum thing was another revelation that completely changed how I thought about aquarium lighting. I’d been using these basic 6500K fluorescent tubes because that’s what the fish store recommended as “full spectrum,” but I had no idea what that actually meant. Turns out, different wavelengths of light do different things for plants. Red light (around 660nm) drives photosynthesis and promotes flowering and fruiting – not that aquatic plants flower much, but it encourages compact, bushy growth. Blue light (around 450nm) also drives photosynthesis and promotes vegetative growth, but too much blue can make plants grow tall and spindly as they try to reach toward what they think is the surface.
Green light, which I always thought was useless since plants reflect it (hence why they look green), actually penetrates deeper into the plant tissue and can drive photosynthesis in lower leaves that aren’t getting much red or blue light. Who knew?
I started experimenting with different color combinations in my classroom tank, and the results were pretty dramatic. When I switched to a light that had more red in the spectrum, my Rotala actually started showing red coloration for the first time. The kids went crazy for it – they couldn’t believe the same plant could be green or red depending on the lighting. Turned into a great lesson about plant pigments and photosynthesis.
The whole adjustable spectrum thing has been a game-changer, though I’ll admit I went a little overboard when I first got a programmable LED fixture. I was changing colors constantly, trying to find the perfect combination, probably driving my fish nuts with all the flickering. My wife walked into the garage one evening while I was programming sunset modes and just shook her head. “Tom, they’re plants. They don’t need a disco show.”
She wasn’t wrong, but man, being able to mimic natural lighting cycles has made such a difference. I program a gradual sunrise starting around 7 AM when I get to school, peak lighting during the middle of the day, and a slow sunset that finishes around 5 PM. The plants seem to love the routine, and honestly, it’s pretty relaxing to watch the tank go through its daily light cycle. Some of my fish have even started spawning more regularly since I implemented the natural lighting schedule.
One thing I wish someone had told me earlier – and this is something I always share with other teachers now – is that more light isn’t always better. I made this mistake with my second classroom tank, thinking that if medium light was good, high light would be even better. Cranked up a powerful LED fixture and promptly turned my beautiful planted tank into an algae farm. Green water, hair algae on everything, black beard algae covering my hardscape. It was a disaster.
The problem was that I’d increased the light intensity without adjusting anything else. Plants need more than just light – they need CO2, nutrients, and proper water circulation to handle high-light conditions. When you give them more light than they can process with the available CO2 and nutrients, they can’t photosynthesize efficiently, and algae takes over. It’s like trying to make a campfire with a blowtorch but only giving it one tiny twig at a time.
That algae outbreak taught me about the concept of limiting factors. In any planted tank, there’s always something that’s limiting plant growth – usually light, CO2, or nutrients. You can’t just max out one factor and expect better results. Everything has to be balanced. If you want to run high light, you need CO2 injection and a good fertilizer routine. If you’re keeping it simple with no CO2, you need to keep the light levels moderate.
These days, my approach to lighting is way more methodical. I start by deciding what plants I want to grow, then research their specific lighting requirements. Carpeting plants like Monte Carlo or dwarf baby tears? They’re going to need high light and CO2. Background plants like java fern or anubias? They’ll actually do better with lower light levels. Red plants generally need more light to maintain their coloration, while most green plants are more forgiving.
I also consider the depth of the tank more carefully now. Light intensity falls off pretty dramatically with depth – it roughly follows an inverse square law, so if you double the distance from the light source, you get about a quarter of the intensity. My 55-gallon classroom tank is 21 inches deep, which means there’s a huge difference between the light levels at the surface versus what’s reaching the substrate. Plants that need high light have to go in the foreground where they’re closer to the fixtures.
The photoperiod – how long the lights are on each day – is another factor I used to completely ignore. I figured more hours of light meant healthier plants, so I was running my lights for 10-12 hours a day. Wrong again. Most planted tanks do better with 6-8 hours of peak lighting. Longer photoperiods just give algae more opportunity to take hold, especially if your plant mass isn’t sufficient to outcompete it for nutrients.
I’ve settled into a routine now where I run my main lighting for about 7 hours, with a dimmer “sunrise” and “sunset” period of about an hour each. The plants get plenty of light for photosynthesis, but I’m not giving algae any extra advantages. It’s worked well across multiple tanks and different plant combinations.
For other teachers thinking about getting into planted tanks, my biggest piece of advice is to start simple and work your way up. Don’t jump straight into high-tech setups with intense lighting and CO2 systems. Get comfortable with low-light plants first – they’re more forgiving, require less maintenance, and you can still create beautiful aquascapes with them. Some of my favorite tanks have been simple setups with basic LED fixtures and hardy plants like crypts, java fern, and moss.
The lighting rabbit hole goes deep if you want it to. You can get into measuring daily light integrals, calculating photon flux densities, analyzing spectral distribution charts… but honestly, most of us don’t need to go that far. Understanding the basics – that plants need sufficient intensity in the right spectrum for the right amount of time – will get you 90% of the way to success.
What I love about aquarium lighting now is how much control we have compared to when I started. LED technology has made it possible to create custom spectrums, program complex lighting schedules, and achieve light intensities that would have required expensive metal halide fixtures just a few years ago. My students are always amazed when I show them how I can control the classroom tank’s lighting from my phone, or when we use the different spectrum settings to observe how plants respond to different colors of light.
It’s been quite a journey from those early days of trusting the watts per gallon rule and hoping for the best. Now when I set up a new tank, I actually know what I’m doing with the lighting – most of the time, anyway. There’s still plenty of trial and error involved, but at least now the errors make sense and I can learn from them instead of just wondering why my plants keep dying.
Tom teaches middle-school science in Portland and uses aquascaping to bring biology to life for his students. His classroom tanks double as living labs—and his writing blends curiosity, humor, and a teacher’s knack for explaining complex stuff simply.
