You know, when people ask me about cube tanks – specifically those 20x20x20 cm ones – I always get this weird mix of excitement and exasperation. Like, I want to tell them how absolutely gorgeous these little tanks can be, but I also want to warn them that they’re going to make you want to pull your hair out at least three times before you get them right.
I’ve been messing around with these cubes for about four years now, ever since my granddaughter saw one at a local aquarium shop and declared it “the perfect size for a fairy garden underwater.” She wasn’t wrong, actually. There’s something magical about a well-done cube tank – it’s like having a tiny slice of nature sitting on your coffee table. But man, the learning curve on these things is steep.
My first attempt was… well, let’s just say it was educational. I figured, hey, I’ve got larger tanks figured out, how hard could a little 8-gallon cube be? Turns out, pretty hard. I made every mistake you can possibly make. Overcrowded it with plants because everything looked so small and cute in the store. Added too many fish because six Ember Tetras seemed reasonable for such a “big” tank. Used regular aquarium gravel because I didn’t think substrate choice mattered as much in a small space.
Within three weeks, I had algae covering everything, my poor little Rotala was melting, and my shrimp were dying off one by one. My husband walked by one morning and said, “That doesn’t look very relaxing,” which was his polite way of saying my tank looked like a green disaster zone. He wasn’t wrong.
The thing about cube tanks is that every single decision gets magnified. In my 40-gallon long tank, if I mess up the plant placement or add one too many fish, it’s not that noticeable. The system can handle it. But in a cube? Every mistake shows. It’s like the difference between cooking for a family reunion versus cooking for two people – you can’t just wing it and hope for the best.
After that first disaster, I stripped everything down and started over. This time I did actual research instead of just assuming I knew what I was doing. Spent probably two weeks reading forum posts from people who’d actually succeeded with nano tanks, watching YouTube videos, taking notes like I was studying for boards again.
The hardscape placement alone took me an entire afternoon. I must have rearranged those three pieces of Seiryu stone twenty different ways before I found an arrangement that didn’t look cramped or fake. In a bigger tank, you can get away with “close enough” on the rock placement. In a cube, it either works or it doesn’t – there’s no middle ground.
I learned to think about scale completely differently. That piece of driftwood that looked perfect in the store? Way too big once I got it home. The rocks I thought would create a nice slope? Turned the tank into a rock pile. I ended up making three separate trips back to my local shop, each time looking for smaller and smaller pieces of hardscape. The owner started joking that I was building a tank for ants.
Plant selection was another adventure in frustration. My usual go-to plants – the ones that work beautifully in larger tanks – either grew too fast, got too big, or just looked completely out of proportion. I had to learn about all these tiny varieties I’d never bothered with before. Anubias nana ‘Petite’ instead of regular Anubias. Eleocharis pusilla instead of the regular hairgrass. Bucephalandra species I couldn’t even pronounce.
The first time I successfully carpeted the front of a cube with Monte Carlo, I was probably more excited than I should have been for a grown woman. But it had taken me three tries and about six months to get it right. The first attempt, I planted it too sparsely and it never filled in. The second time, I planted it too thick and it started dying from the bottom up. Third time’s the charm, right?
Lighting these little tanks is its own special challenge. Too much light and you get algae city. Too little and your plants just sit there looking sad. I went through four different light fixtures on one tank before I found one that worked. The first was way too powerful – I had hair algae growing on everything within a week. The second was too dim – my plants were stretching toward the surface and looking pale and pathetic.
I finally settled on a Chihiros A-Series that I could dial down to about 30% intensity for the first month while everything established. Even then, I only ran it for six hours a day initially. My daughter stopped by during this period and asked if something was wrong with my tank because the light was so dim. I had to explain that sometimes less is more, which felt weird coming from someone who used to think more light was always better.
CO2 injection in these cubes is terrifying, honestly. The margin for error is so small. I nearly gassed all my livestock one night when my CO2 regulator malfunctioned. Came downstairs in the morning to find all my fish hanging out at the surface gasping for air and my shrimp looking distinctly unhappy. Had to do an emergency water change and add extra aeration. Spent the rest of the day testing CO2 levels every hour like an anxious parent taking a sick kid’s temperature.
Now I use a CO2 checker in every cube tank I set up, no exceptions. It’s this little glass bulb filled with indicator solution that changes color based on CO2 levels. Costs about fifteen bucks and has probably saved me from disaster multiple times. I check it obsessively for the first few weeks of any new setup.
Filtration was another puzzle to solve. Regular hang-on-back filters looked ridiculous on these small tanks – like putting truck tires on a bicycle. Sponge filters worked but weren’t powerful enough to create good water movement. I ended up using tiny external canisters for most of my cubes, which was probably overkill but gave me the flow and filtration I needed without cluttering up the visual space.
The bioload management in cubes is critical. I made the mistake early on of thinking that small fish meant I could have more of them. Wrong. Dead wrong. Six Ember Tetras in a 20x20x20 tank might not seem like much, but it’s actually pretty heavy stocking. I learned to think in terms of bioload per gallon rather than just counting fish.
These days, I usually stock my cubes with maybe four small fish maximum, plus some shrimp. My current favorite setup has three Chili Rasboras and a handful of Cherry Shrimp. The rasboras are tiny – maybe half an inch long – but they add just enough movement and color without overwhelming the space. The shrimp are fantastic cleanup crew and add interest without increasing the bioload much.
Maintenance on cubes is frequent but quick. I do 40% water changes twice a week, which sounds like a lot but only takes about ten minutes per tank. The small water volume means that parameters can swing quickly, so I’d rather err on the side of too many water changes than too few. I learned this the hard way after losing a beautiful patch of Glossostigma to a sudden ammonia spike that I didn’t catch in time.
Trimming plants in cubes requires tiny scissors and the patience of a saint. Everything grows into everything else, and you have to trim with surgical precision to maintain the scale and proportions. I’ve got this little curved scissor set that I use exclusively for nano tank maintenance. My husband found me one evening sitting in front of a cube with a magnifying glass, trimming individual stems of baby tears one by one. He asked if I’d lost my mind. Maybe I had, but that tank looked amazing when I was done.
The seasonal changes in cube tanks are more dramatic than in larger systems. When my Monte Carlo starts its winter slowdown, it’s really noticeable because it makes up such a large percentage of the plant mass. When my Rotala puts on its spring growth spurt, I have to trim it every few days or it takes over the entire back half of the tank.
I’ve got five cubes running right now, each with a different theme. One’s an Iwagumi-style setup with just stones and hairgrass. Another is a jungle-style tank packed with tiny ferns and mosses. My newest one is an attempt at a biotope representing a small stream in Malaysia, complete with specific plants and fish from that region.
Each cube teaches me something new about the hobby. They force you to slow down, to really consider every decision, to appreciate the small details that might get lost in a larger tank. When you’ve only got 8 gallons to work with, every plant placement matters, every rock position counts, every livestock choice has consequences.
The funny thing is, these challenging little tanks have made me a better aquascaper overall. The precision and attention to detail they require carries over to my larger projects. I find myself being more thoughtful about plant selection, more careful about hardscape placement, more conservative about stocking levels even in tanks where I have room to spare.
Would I recommend cube tanks to beginners? Probably not as a first tank – the margin for error is just too small. But for anyone who’s got the basics down and wants to really challenge themselves creatively and technically, a 20x20x20 cube is an incredible learning experience. Just be prepared for some failures along the way. They’re part of the process, and honestly, part of the fun.
A retired ER nurse, Elena found peace in aquascaping’s slow, steady rhythm. Her tanks are quiet therapy—living art after years of chaos. She writes about learning, patience, and finding calm through caring for small, beautiful ecosystems.
