Freshwater and Saltwater Aquariums, while having some differences, share many things in common. There is a glass or plastic enclosure that houses the animals, a glass or plastic enclosure that performs a water filtration function, water pump(s) that reticulate the water along with heaters, water purifiers and other devices. The list is a little longer for saltwater aquariums but for now, lets focus on some of the initial similarities and differences.
Aquariums (and filters) -Aquariums are usually made of glass or Acrylic (Plastic) with each medium having pros and cons. Glass is much harder which mean that when you are cleaning, it is much more difficult to scratch. When assembling a Glass Aquarium, Silicone is usually the substance used to seal the edges and bottom together to form a water tight container. Not withstanding a manufacturing defect, a silicone seal should remain intact for 15 to 20 years with the appropriate care. It is because of the hardness of glass that it is my personal favorite after all, the glass is what you actually see first before you see the inhabitance of the aquatic system and if the glass is scratched it will detract from the overall aesthetic appeal. Glass also has some drawbacks. Glass has less optical clarity, when compared to Acrylic, is a limiting factor in how big the aquarium can be (Less than 1000 gallons), will eventually have seal issues and when scratched is much more difficult to buff out. Glass Aquariums are also much harder to create different shapes than Acrylic.
Acrylic Aquariums , on the other hand, have a whole different set of physical properties that are unique and sometimes problematic. Acrylic is amazing in that it can be bent and curved into different sizes, and shapes that greatly expands the aesthetic horizon. When sealing the edges of an aquarium the "Glue" used, bonds the material at a molecular level essentially creating one piece of material. This seal, when done correctly, will never leak. Also, the optical clarity of Acrylic is far superior than glass. The biggest concern I have is that GREAT CARE must be taken when cleaning Acrylic. It is much softer material and can very easily be scratched if you are not paying meticulous attention. It is for this reason that Acrylic is not usually my first choice for aquariums.
Water Pumps -There are really two sub headings under water pumps. Integrated pump/filter combinations such as a "Hang on the back" type or a "Canister" types and separate water pump/filter combinations. Regarding the self contained pump/filter combinations it is almost always more beneficial to get a self contained pump/filter combination that is approximately 25% more than what you need. This is an area, like overfeeding for that matter, that is the one of the biggest novice mistakes. Barring something along the lines of creating a Tsunami in your aquatic system, you can not have to much filtration.
Regarding larger more complex systems, think of a water pump as the heart of your system an as such should be given great consideration when developing plans for your new Aquatic System. Once again, whatever you think you need, get a pump that will deliver 25-50% more gallons per minute. You can always install a valve, or manifold, to cut down the flow. You can even just re-circulate the excess with a loop back to the filter if needed. The old adage that, "It is better to have it and not need it than to need it and not have it" comes to mind here.
Heaters, Carbon Containers, Etc. -Heaters, "Activated Carbon" Canisters and other devices are listed in this category and round out the necessary or complementary devices that make keeping your aquatic system easier. First let me say that I have found that having a heater is usually an option. I say this because most people locate their aquarium in a typical living, or working, space that has an ambient temperature conducive to keeping the usual aquatic inhabitants. Activated Carbon is a must in any aquarium in my opinion. It can be used for removing many impurities that somehow seem to make it into our aquariums. It is a relatively cheap filtration medium that works well in Freshwater and Marine Systems.
Water Purifiers -Water purification is basically removing substances that you do not want in your aquatic system. this can range form macroscopic particles that the naked eye can see all the way down to a molecular level. Various types of filter pads, or cartridges can be used to remove larger particles (Usually measured in microns) while ion exchange mechanisms can filter down to the elemental level. This is where you need to know what you are trying to accomplish. If your test kit has indicated that Phosphates (A negatively charged molecule) are high in your system then you may choose a adsorbent with a positive charge to do the trick. Think of it as magnets in that opposites attract. Although Reverse Osmosis is popular with some folks, my personal favorite is the Kold Ster-il Water Filter, designed and perfected by Ken Howrey of Poly-Bio-Marine, Inc.
The aforementioned is pretty much where the similarities between Freshwater and Saltwater end and although some of the aquatic devices that are primarily used in the Marine Systems will benefit Freshwater Aquariums, their cost effectiveness is sometimes debatable. A good read for those of us that need more in depth information is "Aquatic Systems Engineering: Devices and how they function" by P.R. Escobal. This book IS NOT a light read. I was a heavy science major in college (And continue to do research to this day) and I would not attempt to read this book without my Scientific Dictionary and a Differential Equation Text by my side.