Water, water everywhere, and not a stinking drop to drink. Arguably, water quality is the single most important component in marine aquarium keeping. Quite often the first, best, and only solution to an animal or animals in distress is to execute a massive water change. Therefore, it should go without saying that the quality of the water used to establish the system with, as well as that used to substitute the fouled water during a water change, should be of high quality.
You might think that going to the source, straight to the ocean, would be the answer. Well, sometimes yes, but mostly no. I know, or know of, only a few aquarists that use natural seawater to maintain their tanks. Some do with great results, but I have no experience there and choose not to comment.
"So," you're asking, "how do I go about getting high-quality sea water?" Good question. Most fish stores carry instant seawater; just add water. Actually, there is an abundance of synthetic saltwater mixes available on the market, and most are of high quality. ( If you are interested, Dr. Craig Bingman has written a good article comparing commercial salt mixes. It is located at http://www.aquariumfrontiers.com.) The idea is to pick one and stick with it.
What makes a good saltwater mix? In my opinion, these are the things that I consider when choosing a mix: reputation, approximation of natural sea water, reputation, solubility, reputation, and price. Don't go buy a 150-gallon mix just to get a free T-shirt. Been there. Done that. Got the T-shirt. (Wow, the hits just keep on coming, don't they?) With the T-shirt brand I found that it had reasonably poor solubility, and I suspect that it might have contributed to a phosphate problem, so I had to make a change.
Now, if you've identified a salt mix to use, let's say, oh, maybe… Instant Ocean, you've got to re-hydrate it. Just add water, right? NO. Regardless of your choice of salt mix, your sea water will only be as good as the water you choose to re-hydrate it with. Most community water systems contain some levels of nitrates and phosphates, neither of which we want to add back to the tank. Actually, our goal is to get it out of the tank. That is part of the reason that we are doing the water change in the first place.
A good way to figure out what is in your tap water is to test it, or better yet, obtain a report from your local water company. It should be free of charge, and it will tell you the average levels and thresholds many different things in your water. If nitrate, phosphate, or any other components, like metals, are too high, you are going to have to do something else for or to your water. This is the case with my own water supply.
You can do several things for water. If you choose, and I recommend against it, you can use your tap water regardless of its contents. If you do, you should at least use a de-chlorinator to remove the chlorine and chloramines (basically an ammonia and chlorine bonded atom) from the water first. Unless your tap water is pretty good, though, you might want to go with some form of filtered water.
There are many commercially available water filters. You can get a Reverse Osmosis (RO) and/or De-Ionization (DI) unit from specialty fish-stuff supply companies, like Kent Marine, or from regular old home improvement stores like the Home Depot or Lowes. These can be a little pricey up front, but might be worthwhile in the long haul. Another option is to buy distilled water, not mineral water, from the grocery store, but this could quickly become cost prohibitive. A middle-of-the-road option is a DI unit called the Tap Water Purifier from Aquarium Pharmaceuticals. If you learn to recharge the resins yourself, and a recipe for doing it is posted at http://www.reefs.org, it could get downright cheap.
All right, now we've got the salt and the water to re-hydrate it. What is our goal here? We need to mix the water and the salt to achieve a specific gravity, or less directly salinity. Either way, we want to know how much salt is dissolved into the water. The target for specific gravity is generally accepted as between 1.021 and 1.026 for marine fish, and toward the higher end of that range for corals and invertebrates.
Generally, a hydrometer is used to measure salinity. Hobbyist hydrometers are usually nothing more than a plastic box with a floating needle and a graduated scale. They are notoriously inaccurate, but pretty easy to use and inexpensive. Floating glass hydrometers are a little more accurate and a little more expensive. (For you home-brewers out there, you can use your home-brew hydrometer, just clean it off well.) Electronic devices are deadly accurate, but cost quite a few bucks.
If you can't afford the Cadillac-electronic-bad-to-the-bone- techno-probe, don't fret. It is not the specific gravity that is so important, but that you consistently maintain your water at whichever level you adopt. So, if your hydrometer is off by .002, but you consistently maintain your salinity at 1.024, your animals will be fine. If it wanders all over the board, the osmotic changes will eventually stress your livestock, particularly the invertebrates.
Another couple of important targets to obtaining primo salt water should be discussed: pH and alkalinity. I won't pretend to understand the complexities of pH, other than to say that the physiological functions of marine animals perform at optimum levels within a very narrow range of pH. Therefore, maintaining a stable, high pH of around 8.3 is in the best interest of the animals. Bob Fennner's book, "The Conscientious Marine Aquarist", has a good discussion of pH for the non-chemist.
Alkalinity is a measurement of the buffering capacity of the water. This buffering capacity allows the system to resist the tendency of pH to dive, particularly in a system high in dissolved organics. By the way, pH and alkalinity are easily measured with commercial hobbyist test kits, just the same as ammonia, nitrite, and nitrate.
Let's go back to our salt mixes. Good commercial salt mixes easily achieve the pH and alkalinity requirements, and frequent water changes with same will help you maintain it without the need to add additional buffers and such.
Oh, almost forgot. Temperature needs to remain relatively stable, just as salinity and pH. This is not really a component of the water as much as it is a property of the water. Arguments will ensue if I say that X° F. is optimum for a reef aquarium. But I think the acceptable range is between 72° and 85° F. I shoot for a good solid 80°, but that is just me. Again, stability is the key.
To keep your water warm, you need a good heater. Don't get a cheap one because they are prone to short out and die. Get two high-quality heaters in the 3 watts-per-gallon range. The second is in case the first one fails, you have a standby. In Fairbanks, Alaska this is particularly important.
In summation, water quality could very well be the single most influential component of a successful aquarium, fresh or salt, fish or invertebrate. Take care to do this part right. Pick a good reliable salt, say… uh… well…Instant Ocean, mix it with good fresh water, and do frequent partial water changes.
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