Wet Dry or Sump — Which One Should You Get?

Wet dry or sump — it's one of the most common questions we get at Nature Aquariums. Both do similar things, but choosing the wrong one for your setup can make your life harder down the road. In this video, Henry breaks down the differences, explains which one is right for your tank, and walks through water changes, fill levels, and evaporation top-off — everything you need to run either system correctly.

What They Have in Common

Both a wet dry and a sump serve the same two basic purposes: they add extra water volume to your system and give you a place to hide your equipment — heaters, protein skimmers, media reactors, CO2 systems, Apex controllers, and more. A 20-gallon sump on a 100-gallon tank effectively gives you 120 gallons worth of water stability. That extra volume matters.

The Key Difference

A wet dry uses bio balls — the "wet and dry" area where water trickles through, allowing beneficial bacteria to colonize and convert ammonia to nitrite, and nitrite to nitrate. It's a tried-and-true biological filtration method that's been around for decades. Water then passes through a sponge block to reduce bubbles before the return pump sends it back up.

A sump is essentially a chambered tank. On its own it has no biological media — it's designed to hold equipment and increase water volume. However, you can add appropriate media (like Matrix, Sera Siporax, or similar) to a sump's refugium chamber to handle biological filtration, making it work similarly to a wet dry.

Which One Should You Use?

Henry's rule of thumb:

Freshwater tanks → go with a wet dry. Efficient, easy to clean, great ammonia reduction, simple to maintain.
Saltwater/reef tanks → go with a sump. Live rock in the display handles biological filtration, and the sump gives you the room you need for skimmers, algae scrubbers, and other reef equipment.

If your tank came with a built-in sump — like a UNS, Red Sea, or ProClear system — those were designed for saltwater. You can convert them for freshwater use by filling the refugium chamber with biological media, but you'll need to plan your chamber layout carefully.

If you're running a more complex freshwater setup with an algae scrubber, Purigen reactor, CO2, and heavy heating — a sump may make more sense than a wet dry, just make sure you choose one with a chamber that can hold biological media.

How to Do Water Changes with a Sump or Wet Dry

The process is the same whether you're running a sump or a wet dry:

Turn off your return pump. Water will begin draining from the tank back into the sump until it hits the bottom of the weir — that's the siphon break point.
Before you start draining, mark your water line with a piece of masking tape. This is your target fill level when you're done.
Do your water change and gravel vac as normal.
When refilling, add water to the top tank — not the sump. The sump is already full from the drain-down. Fill the display tank back up to your tape mark.
Turn the pump back on. The excess water in the sump will pump back up and everything levels out.

Sump Sizing — Don't Go Too Small

This one catches a lot of people off guard. When your return pump shuts off, all the water sitting in your plumbing and above the weir drains back down into the sump. If your sump is too small, it will overflow. Henry's advice: size your sump to match your tank, and if you have the room and budget, go one size up as a safety buffer. A 125-gallon tank should not be running a sump rated for 75 gallons.

Proper Fill Level & Handling Evaporation

When setting up for the first time, fill your display tank until water flows over the weir and into the sump. Find your return pump chamber and set your normal operating level at about 2 inches above the pump intake. Mark that level — it's your baseline.

As water evaporates, the level will drop in the return pump chamber only — not in the display tank or other sump sections. Top off with RO water (saltwater tanks) or dechlorinated water (freshwater tanks) to bring it back to your mark. It doesn't matter whether you add it to the display or the sump — it will find its own level.

Never let the level drop so low that the pump starts drawing air and creating a vortex. And never fill so high that when the pump shuts off, the drain-down water overflows your sump. That buffer zone is your safety net.

Video Chapters

0:00 — Introduction: wet dry vs. sump
0:15 — What they have in common: extra water volume & equipment storage
0:55 — How a wet dry works: bio balls, denitrification, bacteria
1:35 — How a sump works: chambers, media options, biological filtration
2:20 — Rule of thumb: freshwater vs. saltwater use cases
2:40 — Converting built-in sumps (UNS, Red Sea, ProClear) for freshwater
3:15 — Simple freshwater setups: why wet dry wins
3:40 — Complex freshwater setups: when to choose a sump
4:30 — Overflow tanks: works for both fresh and salt
4:45 — Return line setup & oxygenation tips
5:15 — Maintenance: cleaning weirs, siphon breaks explained
5:45 — Live demo: UNS R150 converted to freshwater sump use
6:40 — Water change walkthrough: step by step
7:30 — Sump sizing: why proportions matter
8:40 — How to mark your water line for refilling
10:30 — How to properly fill a sump for the first time
11:30 — Evaporation top-off: where the level drops & how to fix it
12:35 — Pump intake level, air vortex prevention, overflow safety
13:30 — Filter pads, sock filters & roller mats: manufacturer's choice
14:25 — Wrap-up & call for video suggestions

Bottom Line

Neither a wet dry nor a sump is universally better — it comes down to what you're building. Freshwater with lots of fish and hardscape? Wet dry, all day. Reef tank or complex freshwater system with lots of equipment? Go sump, just make sure it can hold media. Size it right, mark your levels, and you'll have a system that's easy to maintain for years.

We carry wet drys, sumps, and everything you need to set them up right at natureaquariums.com — shipped right to your door.