How to Convert a Freshwater Aquarium to a Saltwater Aquarium
For some freshwater aquarists, making the switch to a saltwater system can seem like a daunting task. However, aside from a few minor differences, keeping a saltwater aquarium is really not that difficult as compared with a freshwater aquarium. There are a few things to consider, so let's take a look at those and learn how to convert a freshwater aquarium to a saltwater aquarium.
Start at the bottom and work your way up. The first thing you'll need to do is remove the freshwater substrate (gravel) and under gravel filter is using one. You will want to cover the bottom with crushed coral or aragonite sand that is made for marine aquariums. While you are at it, remove any wire based heaters under the undergravel filter. Most any other type of heater should be suitable as long as it is rated for use in both freshwater and marine environments, you should be able to use your existing heater.
Plastic plants, or decorations such as pirate chests, sunken ships or Spongebob Squarepants statuettes. While they may be fun to look at, they have no place in a proper marine aquarium. Replace the plastic and resin decorations with live rock. This will be a big part of your biological filtration system in your marine aquarium whether it is a FOWLR or reef tank, you will need live rock. Take some time and research some aquascaping, and you can come up with some pretty neat ways to stack live rock to emulate natural reefs.
Which brings us to filtration. While the live rock will perform a large amount of your biological filtration, you should also employ mechanical and chemical filtration. Luckily most fresh water aquariums already have hang on tank (HOT) filters and or canister filters. Marine fish and invertebrates require better quality water than freshwater livestock, so you may find that your filters and filter media require more frequent cleaning and changes. Alternately, you can also upgrade your filtration by adding an additional HOT or cannister filter, or by adding and overflow box and a sump. If you want to convert to a reef aquarium, you will also need to get a protein skimmer. They are available as in sump or hang on tank models. Be sure to select one that is rated for your aquarium size.
Water flow is also more important in marine aquariums, especially so for reef tanks. If you have a power head it will work in your saltwater aquarium. However, if you are looking to convert to a reef tank, you may need to get additional power heads and a wave maker.
Lights are the last main component of your aquarium to consider, and are perhaps the most important. While low wattage fluorescent bulbs and LED lights are fine for a saltwater fish only or a marine Fish Only With Live Rock aquarium, if you want to keep corals you will need bright, intense lighting that is suitable for corals.
You will also need to get salt mix or purchase pre-made saltwater from your local fish store. Most saltwater aquarists prefer to make highly filtered RO/DI water and then mix their own saltwater in a separate mixing tank so they have at least 55 gallons or more on hand at any given time. This is especially true if you want to keep a live coral aquarium.
As far as new equipment you must get, a hydrometer or salinity refractometer is essential so you can tell what the specific gravity of your saltwater is, and you will need a marine basic water test kit to monitor the pH, ammonia, nitrite, nitrate and phosphates while your new salt water aquarium is cycling.
One last bit of advice, do not overstock your marine aquarium. While the rule of thumb for freshwater aquariums is 1" inch of fish per gallon, marine aquariums require better water quality so try to stick to a 1" of fish per 5 gallons rule of thumb for marine systems.