How to Maintain Salinity in a Desktop Aquarium
- PNW Custom
- 4 days ago
- 3 min read
Maintaining a stable salinity is one of the most important parts of keeping a saltwater aquarium healthy, especially for desktop aquariums. In tanks under 5 gallons, even a little bit of evaporation can cause salinity to creep upward, which stresses corals and invertebrates. The good news? With the right habits, keeping salinity stable in a tiny tank is easier than you might expect.
Whether you’re running a 40oz Micro Tank, a Small-In-One, a 4-Gallon Mini Pond, here’s how to maintain your salinity every day.
Why Salinity Stability Matters
When water evaporates, the salt stays behind. This means that the salinity level rises unless you replace the lost water with pure RODI. Thankfully, PNW Custom aquariums are designed to limit those swings from the start.
Our Tip: We recommend using a salinity refractometer so that you always have an accurate reading of your salinity. Aim to be within the 1.024–1.027 range.
Evaporation-Control Lids: Your First Line of Defense
Every PNW Custom desktop aquarium includes a precision-cut, polycarbonate lid designed to dramatically reduce evaporation. A lid is a powerful tool for keeping small aquariums and is one of the biggest reasons our tanks stay stable even without extra equipment:
Less evaporation = fewer salinity swings
More stability = happier corals and fish
No need for constant top-offs
Our Tip: Wondering how often you should top off your aquarium? Use our FREE Weekly Maintenance Checklist as a guideline so you can start getting into a consistent routine.
Built-In Engraved Water-Level Indicator
Another PNW Custom feature that makes salinity management incredibly easy is our Engraved Water-Level Indicator, included on many of our desktop aquariums. This simple but helpful visual guide shows your tank’s ideal water height, so you always know exactly when to top off. This helps prevent both overfilling and under-filling and gives you an important baseline to monitor your water level.
Optional: Use an ATO for Automatic Stability
While topping off manually is perfectly manageable thanks to the reduced evaporation in our tanks, some hobbyists like the extra convenience of automation.
An Auto Top-Off (ATO) system refills evaporated water automatically throughout the day. PNW Custom offers a compact ATO and matching ATO reservoir that fit perfectly on a desktop, ideal for those who travel, have irregular schedules, or simply want that “set it and forget it” stability.
Our Tip: For beginners, we recommend starting without an ATO. This keeps things simple and less equipment-heavy until you are more in-tune with your tank. Start with checking salinity daily and topping off when needed.
What Not to Do: Common Mistakes
Avoid these common beginner pitfalls:
Don’t ignore evaporation. Even with lids, it's still important to check the tank regularly.
Avoid big corrections. If salinity does rise, bring it down slowly by topping off gradually with RODI.
Don’t guess. Measure salinity, your eyes can’t tell the difference.
Top off with RODI water. Only top off with saltwater if your salinity level is low.
Weekly Water Changes
Regular water changes naturally help maintain stable salinity, replenish minerals, and keep parameters in check. Even a small weekly change (30%) goes a long way in desktop aquariums.
Our Tip: Consistency + weekly water changes = long-term stability.
Stability Made Simple
Maintaining salinity doesn’t need to be stressful, especially with a tank built for managing it. Thanks to the evaporation-control lids and filtration included with every PNW Custom aquarium, your desktop reef is already set up for long-term success. Manual top-offs are easy, fast, and totally manageable, and if you ever want automation, our ATO + ATO Reservoir give you an optional upgrade path. Stable salinity = stable reef. And with a routine, your tiny tank will stay thriving, colorful, and happy for years.
