One thing you need to know about brackish water is that it’s not simply either freshwater or seawater (ocean). There’s a range of water in nature. There’s brackish water that falls somewhere in-between those categories. Keep that in mind. You are going to have to change how you think, since what you’re caring for isn’t strictly freshwater and it also isn’t full-out marine. It’s in-between so you’ll want to adapt your care accordingaly.
Specific Gravity; This is a measure of water density compared to pure water. On that chart (above) I broke it down by specific gravity which is the number comparing water density to pure water. The ocean floats right around 1.025 and higher. Freshwater are pure with a reading of 1.000. So fish adapt their physiology for this range between the two extremes, where brackish water is middle ground.
Basic Tips for Keeping Brackish Fish
Salts change osmoregulation, so you need to match the salinity to the fish. For instance, putting a low brackish fish in full marine strength water will stress it out. Similarly, keeping a high brackish species in fresh water will likely cause them to fail to thrive over time. If this is your first time keeping brackish species, begin with easy ones.
Some gobies such as bumblebees is tolerant of low salinities. In fact, they can tolerates straight fresh water on occasion. Salt also tends to make them more colorful and healthy. Figure eights (see infographics above) do better in mid-brackish ranges. These are interesting species but they’re demanding… They require hard-shelled food to maintain their beak-like teeth. If they don’t get snails or clams in their diet, for instance, their teeth will overgrow. This sounds cruel but it’s biology. They’ll literally starve to death while having food right in front of them.
Equipment requirements shifts somewhat as you go up in salinity, i.e., you have to be able to see what you’re doing! You’ll want something that’s reasonably accurate and inexpensive for this purpose, such as a hydrometer if being in the ballpark is fine. Better still would of an optical refractometer that provides very precise measurements without any temperature compensation headaches. Again, being accurate means avoiding accidental shocks from a water change.
Always use fresh, dechlorinated tap water or RO/DI water to replace evaporated water. The salt will remain behind when water evaporates, so your specific gravity increase and any toxins you don’t keep track of become more concentrated. Consider mangrove roots (not just plastic plants) and try to duplicate that physical arrangement as much as possible. Think “estuary.” Real mangrove propagules (roots) can handle saltier water. Java fern and Anubias does fine in lower brackish levels. Mangroves serve as cover, and many brackish fish is naturally territorial, which means they are aggressive, so these plants break their line of sight.
Because archer fish spit water at prey located above the tank to dislodge them, ensure they have space and make sure your tank has a solid lid. Mudskippers will jump at the opportunity, so secure hardware is a must with this one.
This compatibility chart takes all of the guess work out of keeping fish. Trying to maintain two species together, one that requires high salinity and another that prefers low salinity; means heartache. Make your selection (high salinity, mid-salinity or low salinity) and stay within that zone. This will make maintaining the group much easier. After selecting your salinity window, tweak the tank parameters accordingly and allow the colony to stabilize. Remember, perfect is the enemy of good in this hobby, stability wins every single time.
You get to witness behaviors not witnessed anywhere else. Puffers that intensely investigate each nook and cranny and gobies that dig into the sand. There’s a feeling of witnessing a small world of tidal life rather than watching your pet. You’ve created a transition zone where most folks speed by to hit the beach. And that’s what it’s about, that discovery, so the labor is rewarded in there every small movement.
