Most hobbyists find that the stock fluval evo 13.5 filtration is a decent starting point, but it quickly becomes the first thing they want to upgrade once they get serious about reefing. It's a sleek, all-in-one system that looks great on a desk, but the way the back chambers are set up out of the box isn't exactly ideal for long-term water quality. If you're tired of looking at cloudy water or fighting constant algae blooms, tweaking how your water flows through those back compartments is the best move you can make.
What's Actually Happening Back There?
The back of the tank is divided into three distinct chambers. In the standard out-of-the-box setup, water flows through the intake grates into Chamber 1, travels through a massive black sponge in Chamber 2, and then gets pumped back into the display from Chamber 3. It sounds simple enough, but there are a few design quirks that make this less efficient than it could be.
The biggest issue is the "bypass" hole. If you look into the second chamber, you'll see a small circular hole cut into the plastic wall. Fluval put it there as a safety feature so the pump doesn't run dry if the water level drops, but it actually allows a lot of dirty water to skip the filter media entirely. Most experienced owners end up plugging this hole with a bit of aquarium-safe plastic or a specialized plug you can buy online. By closing that gap, you force every drop of water to actually go through your filter media, which is exactly what you want.
Rethinking the Giant Black Sponge
When you first open the box, you'll see a large foam block with two cutouts for ceramic rings and activated carbon. While it works for a while, that sponge eventually becomes what we call a "nitrate factory." It's great at trapping fish waste and leftover food, but because it's so large and dense, it's a pain to clean. Most people find themselves getting lazy with it, and that trapped gunk just sits there and rots, fueling hair algae and spiking your nitrate levels.
Instead of keeping that giant sponge, many reefers switch to a media basket. These are usually made of acrylic and fit perfectly into Chamber 1 or Chamber 2. Using a basket allows you to stack your filtration in layers that are much easier to swap out. You can put some cheap filter floss at the very top to catch the big particles, then layer in your chemical and biological media underneath. When the floss gets brown and nasty—which usually happens in a few days—you just toss it and put in a fresh piece. It takes ten seconds, and your tank stays way cleaner.
Optimizing Chamber 1 and Chamber 2
If you decide to go the media basket route, Chamber 1 is usually the best place for your mechanical filtration. Since this is where the water first enters from the display, putting your filter floss here ensures that the "big stuff" gets caught immediately. Some people also like to run a small bag of high-quality activated carbon here to keep the water crystal clear and remove any toxins corals might release.
Chamber 2 is where you have the most room to play. If you've moved your mechanical filtration to Chamber 1, you can use this middle space for biological media like ceramic rings, Bio-Balls, or even a small bag of crushed coral. Some people even turn Chamber 2 into a tiny refugium by adding a submersible LED light and some macroalgae like Chaetomorpha. The algae eats up phosphates and nitrates, acting as a natural scrub for your water. It's a bit of extra work to maintain, but it's a very effective way to keep your parameters stable.
The Role of the Protein Skimmer
You might be wondering about the Fluval PS2 protein skimmer designed specifically for this tank. It's designed to fit into Chamber 1, but honestly, it's a bit of a polarizing piece of gear. Because the fluval evo 13.5 filtration area is so small, adding a skimmer can be a tight squeeze.
In a tank this size, frequent water changes (about two gallons a week) usually do a better job of removing waste than a small, finicky skimmer will. However, if you plan on stocking the tank heavily with fish or you really want to push the limits of what the tank can handle, a skimmer can help. Just keep in mind that they can be noisy and require a lot of "dialing in" to get that dark, nasty skim mate instead of just clear water.
Boosting the Flow
The stock return pump that comes with the Evo 13.5 is okay, but it's not exactly a powerhouse. Good filtration isn't just about the media you use; it's about how much water you can move through that media. Many hobbyists upgrade the stock pump to something like a Sicce Micra Plus. It fits in Chamber 3 perfectly and provides a much more vigorous flow, which helps keep detritus from settling on your rocks and sand.
Another popular tweak is adding a "Random Flow Generator" nozzle to the return outlet. The stock "duckbill" nozzle just shoots water in one direction, creating dead spots where waste can collect. A random flow nozzle spins the water around, mimicking the natural movement of the ocean. This keeps the waste suspended in the water column longer, giving your fluval evo 13.5 filtration a better chance to actually suck it up and filter it out.
Maintenance Tips for Long-Term Success
No matter how many upgrades you buy, the filtration only works if you stay on top of it. Because the Evo 13.5 is a small volume of water, things can go south pretty quickly if you neglect it.
I've found that the best routine is to change the filter floss every three or four days. If you leave it for a week, it starts to break down and actually makes your water quality worse. You should also take the time to rinse out your ceramic rings or other bio-media in a bucket of old tank water (never tap water!) about once a month. This knocks off the slime and gunk without killing the beneficial bacteria that keep your fish alive.
Don't forget about Chamber 3, where the pump sits. Over time, salt creep and debris can build up in there. Every few months, it's a good idea to pull the pump out entirely and give it a good vinegar soak to keep it running smoothly. This is also a good time to wipe down the walls of the back chambers to prevent any nasty buildup.
Final Thoughts on the Setup
At the end of the day, the fluval evo 13.5 filtration system is what you make of it. If you leave it completely stock, you can still have a nice tank, but you'll be working a lot harder to keep it looking pristine. By making a few smart changes—like plugging the bypass hole, switching to a media basket, and upgrading the pump—you turn a basic kit into a high-performing reef system.
It's all about making the maintenance easier for yourself. When the filtration is efficient and the media is easy to change, you're much more likely to keep up with it. And in the world of saltwater tanks, consistency is the real secret to success. Whether you're keeping a simple pair of clowns or a full-blown "SPS" coral garden, getting that back-chamber setup dialed in is the best investment you can make for the health of your little underwater world.