Calcium: Calcium is a mineral that is essential for strong bones and teeth. It is also important for muscle function and blood clotting. The amount of calcium in drinking water typically ranges from 10 to 200 milligrams per liter.

Magnesium: Magnesium is a mineral that is involved in many bodily functions, including energy production, protein synthesis, and nerve function. The amount of magnesium in drinking water typically ranges from 5 to 50 milligrams per liter.

Sodium: Sodium is a mineral that is important for fluid balance and nerve function. However, too much sodium can be harmful to people with high blood pressure. The amount of sodium in drinking water typically ranges from 20 to 200 milligrams per liter.

Potassium: Potassium is a mineral that is important for muscle function and blood pressure control. The amount of potassium in drinking water typically ranges from 1 to 5 milligrams per liter.

Chloride: Chloride is a mineral that is important for fluid balance and stomach acid production. The amount of chloride in drinking water typically ranges from 100 to 400 milligrams per liter.

Sulfate: Sulfate is a mineral that is important for the body's metabolism. The amount of sulfate in drinking water typically ranges from 20 to 200 milligrams per liter.

Fluoride: Fluoride is a mineral that can help to prevent tooth decay. The amount of fluoride in drinking water is typically controlled by the water utility to a level of 0.7 milligrams per liter.

Carbon Dioxide: Carbon Dioxide is a gas that is added to drinking water to make it taste better. The amount of carbon dioxide in drinking water typically ranges from 2 to 3 milligrams per liter.

In addition to these chemicals, drinking water may also contain small amounts of other chemicals, such as lead, arsenic, and mercury. These chemicals can come from the natural environment or from human activities, such as industrial pollution. The levels of these chemicals in drinking water are typically very low and are not considered to be a health risk to humans - but the same is not true for your fish.

On top of what's residing in the tap water from the water source, your municipality will also treat the water to ensure it's safe for human consumption. Again, these additives and processes may still be harmful to your fish.  The most common methods of water treatment include:

• Chlorination: Chlorine is used to kill bacteria and viruses in the water.

• Fluoridation: Fluoride is added to the water to help prevent tooth decay.

• Sedimentation: Sedimentation is a process that allows larger particles to settle out of the water.

• Filtration: Filtration removes smaller particles from the water.

• Disinfection Byproduct Control: Disinfection byproducts are formed when chlorine reacts with other chemicals in the water. The city of Plano uses a variety of methods to control the formation of these byproducts.

So you're probably thinking: I've got to fill my tank with water - what are my options?

Getting water into your tank varies significantly for freshwater tanks versus saltwater tanks.  Since saltwater is the bigger challenge, we'll start there and use some of these lessons when we discuss freshwater.

There are three primary ways to get water into your tank:

1. Treated Tap Water

2. RO/DI Filtered Water

3. Purchased Water

Let's look at each in more detail:

Treated tap water - Treated tap water is not recommended for Saltwater tanks; although it will work, and there are those who are successful with this method, the treatments only mask the contaminates, which remain in the water bonded to another chemical agent that prevents damage to the fish.  While these chemicals are effectrive and can be trusted, the water remains loaded with contaminates that can cause algae blooms, diatoms, and other unwanted trouble.  Avoid this method unless you're simply unable to use 

Filtered water - Saltwater for fish consumption should be pure water - as pure as possible with as few additives or chemicals as possible.  This, thankfully, is not quite as challenging as it sounds with the advent of home Reverse Osmosis, Deionization (RO/DI) systems.  

A reverse osmosis deionization unit works by forcing water through a semi-permeable membrane. This membrane allows water molecules to pass through, but it blocks most other molecules, including dissolved salts, minerals, and bacteria. The water that passes through the membrane is called permeate, and it is very pure. The water that does not pass through the membrane is called reject, and it contains the contaminants that were removed from the permeate.

A reverse osmosis deionization unit typically has three stages, but in our aquarium hobby we frequently use 5 to 7 stages to get even more pure water and to use as much of the water from the source as possible.  Traditional 3 stage RODI will typically create a permeate to reject ratio of 1:3, meaning three units of water are disposed of for each unit of pure water kept.  By adding an additional RO pass, we're able to bring that ratio to 1:1 or even 2:1.  

1. Pre-filtration: This stage removes large particles, such as dirt and sediment, from the water.

2. Carbon Filtration: This stage removes more particles, odors, and tint from the water.

3. Reverse osmosis: This stage removes dissolved salts, minerals, and bacteria from the water.

4. Reverse Osmosis 2nd pass: this is a scavenge filtration pass that looks to create more permeate and less reject.

5. Deionization: This stage removes any remaining ions from the water, making it even more pure.  This is an important stage for anyone who lives in a municipality that uses chloramines to disinfect drinking water instead of just Chlorine. Chloramines are a combination of chlorine and ammonia. They are used to disinfect drinking water and swimming pools. Chloramines last much longer in water than chlorine alone, which is prone to evaporation.  Deionization is one of the few methods to remove chloramines from you water.

6. Some systems will add a second carbon filtration and a second dionization stage.

The water that comes out of a reverse osmosis deionization unit is very pure, but you will still need to add salt to your RODI water to create saltwater with a specific gravity of 1.026.  Once your water has been filtered through the RODI, salt is added and mixed per the manufacturer's recommendations.  We will talk about salt in a future article.

Making your own RODI water is cheaper than buying it, but it's still expensive; looking at my local area, here's the calculations:

The cost of making RODI permeate using a 5-stage RODI system in Plano, TX can be calculated as follows:

Cost of filtration media per gallon of permeate = Total cost of filtration media / Number of gallons of water the filtration media can process = $500 / 5,000 gallons = $0.10 per gallon

Cost of water per gallon of permeate = Price of residential water in Plano, TX / Number of gallons of permeate per gallon of water = $1.25 / 2 = $0.625 per gallon

And even though I'm throwing this water away as waste, the cost of water per gallon of reject = Price of residential water in Plano, TX / Number of gallons of reject per gallon of water = $1.25 / 2 = $0.625 per gallon

Total cost of making each gallon of RODI permeate = Cost of filtration media per gallon of permeate + Cost of water per gallon of permeate + Cost of water per gallon of reject = $0.10 + $0.625 + $0.625 = $1.35 per gallon

Assumptions

• The RODI unit can process 5,000 gallons of water before the filtration media has to be replaced.

• The total cost for the filtration media are $500 per lifecycle.

• The price of residential water in Plano, TX is $1.25 per gallon.

Notes

• The cost of making RODI permeate can vary depending on the price of water, the cost of filtration media, and the efficiency of the RODI unit.

Since I have two 300 gallon tanks in my home at the time of this writing, and since I do my weekly 10% water changes, I'm spending just under $6,000 a year in water and salt:
I replace 60 gallons of water each week. This means that I will replace 4200 gallons of water in one year, then the total cost of replacing the water in your aquarium using RODI water will be $5220.  Add to this the cost of salt at $500/year, I spend $5,720 to keep up with my water changes.

Here is a breakdown of the costs:

• Cost of water: $1.35 per gallon x 4200 gallons = $5220

• Cost of salt: $500

• Total cost: $5720

Purchased Water:

As noted, making your own water with filtration and salt will be cheaper than buying saltwater from a local fish store (LFS) - but it comes with much fewer headaches, time, and energy.  Whether the tradeoff is worth it to you is a decision you'll have to make, but by comparrison, aquarium ready saltwater in my area varies between $2.59 to $2.99 per gallon.  In addition to the increased cost, you'll also need to procure water jugs that allow you to collect and transport the saltwater from your local retailer to your home.  Plus gas, time, and frustration.  The same 4,200 gallons of water that I changed above would now cost me as much as $12,558 per year!  That's quite a premium for convenience.

Freshwater tanks:

We're about to do a 180 degree turn, so hang on:  Freshwater tanks are primarily filled with tap water, treated with a water conditioner that binds the chlorine and keeps it from harming your fish.  You might ask if you could use RODI water in your freshwater tanks - and the answer is NO!  NEVER USE DEIONIZED WATER in a freshwater tank - it's too pure and lacks the necessary minerals the fish need.  It also can result in health problems such as osmotic shock, which can be fatal. Note: these same minerals are added into saltwater via the salt mix, thus it's not detrimental in a saltware tank.

Osmotic shock occurs when the water inside a fish's body has a different concentration of minerals than the water outside its body. This can happen when fish are placed in deionized water, which has no minerals. The water inside the fish's body will try to move to the deionized water in an attempt to equalize the concentrations of minerals. This can cause the fish's cells to swell and burst, which can be fatal.

There are some who have recommended using RO water with mineralization added back to the water, but given the additonal costs and potential risks, I do not use this method in my own freshwater tanks and I do not recommend it to others.