Let's look at these benefits in order.

The larger the amount of water in your tank, as a rule of thumb, the better because of the ability to dillute bad things.  Think of the ocean - where for generations humans have dumped so many horrible things from trash, toxic pollutants, even radiation, and our oceans still are teeming with life.  This is not an endorsement for past behavior and I'm seriously, seriously against dumping anything into our oceans, or our tanks.  But, the truth is that more water allows for a little more tolerance before mistakes become fatal to your fish and corals.  A Display Tank (DT) that is 300 gallons with an additional sump volume of about 100 gallons is really a 400 gallon total system.  Simple enough, right?  

In addition to more water volume, this is also a great place to introduce chemicals, beneficial bacteria, etc., that is then dilluted in your sump before getting introduced into the display tank with your fish and corals.  Dilluting before introduction into your tank is always a good idea unless the instructions on your product specifically indicate to add directly.

What about filtration options?  What does this mean?  As we mentioned in What Kind of Aquarium Hobbyist Are You? there are a lot of options to choose from - but in every case they all have common requirements: we have to remove the toxins and replenish the oxygen in the water to keep our fish alive and happy.  This filtration can include a dizzying array of options, from filter floss to micron sized filtration pads to remove detritis, but this is only the first stage.  A good filter will also include a place to house beneficial bacteria that do the hard work of Nitrogen cycle.  Additional filtration can include carbon to remove both smells and toxins, Granular Ferric Oxide (GFO) to combat Phosphates, Biopellets that provide a solid carbon food source for the denitrifying bacteria, and even highly specialized filtration for removing copper, nitrates, and other undesirable toxins in our tanks.  

All these complex systems require fairly regular maintenance and cleaning.  Having an open sump means you're able to reach in and clean without disassembly.  This also gives you a place to house a refugium for those who are maintaining a reef tank.

Given the flexibility and benefits of a sump, why doesn't everyone use one?
There are a few downsides to adding a sump to your aquarium.

• Cost: Sumps can be more expensive than traditional filters.

• Complexity: Sumps can be more complex to set up and maintain than traditional filters.

• Space: Sumps require additional space.

• Risk of leaks: If the sump is not properly sealed, it can leak water into the room or onto the floor.

We can again look at each of these, but before we do, let's take a quick look at the alternatives. 

If you've chosen to not use a sump, what are your options?  

So, what are the downsides to having a sump? 

Cost is not trivial.  Commercial sumps will vary in price depending on volume, features, and brand.  For my 300 gallon tanks, the cheapest new sump I found (and bought) was just under $700.  For the Bashsea sump  I  wanted, I was looking at over $1,200 for the optimal size.   Do note that you do not need to spend this much on a sump box.; my second 300 gallon tank has been running for 2.5 years using a 55 gallon aquarium bought on a super sale ($1/gallon) from Petco and then converted into a sump by glueing in acrylic pannels to create separate chambers to fulfill the same needs as the pro version.  There are tradeoffs, though; for example, this is not the sump I'm showing off to my friends when they visit.  Your mileage may vary.

What about complexity?  For the most part, this isn't that unmanageable.  You will be required to plumb your sump - meaning you're moving water from the display tank to - and through - the sump, and then pumped back into the display tank.  Plumbing options vary between hard plumbing (PVC) or soft plumbing (Flexible hose), with the former having a distinctly more professional and thus desirable appearance.  You'll also need to figure out where you're going to place your equipment in the sump, whether or not you're be incorporating a refugium in your sump (see below), whether you'll be using automation for your chemistry and tank monitoring, etc.  In my sumps I have the following setup:

• Filter socks - a first stage filtration intended to trap the largest particles coming from your display tank.  Water is directed through these socks in the first chamber, with the water exiting the socks and then through an opening at the bottom of the first chamber into my skimmer chamber.  Socks are cleaned 2 - 3 times per week.

• Skimmer - I use skimmers on every tank I own - freshwater or saltwater.  They're relatively cheap and they do an admirable job of removing nitrifying waste from the water column.  They need to be cleaned weekly to biweekly and the remnants called skimmate stink something ungodly, but they're worth it.  I also include my heater in this chamber, farther away from my temperature probe to make sure I'm getting an accurate reading.

• Refugium - A refugium is a chamber in your sump, or a separate sump, that is specifically designed to grow macroalgae.  Algae is a love hate relationship to the aquarium hobbyist, but in my opinion they do some pretty amazing things.  The algae we grow in our refugium helps to remove nutrients from the water and improve water quality; but even more impressive is that by picking which algae we grow in our refugium, we can also target things like removing copper from the water column, or other toxic metals.  Truly, algae is the reason that life exists on our planet.  

• Bio balls and an algae reactor - bio balls are extremely porous rock (also available in bricks, or designer shapes) that allow water to pass through easily, but give bacteria a home to live in.  The algae reactor is there to try to outcompete the undwanted algae.  We'll cover algae a lot in a future post.

• Probes and monitoring equipment - I monitor several parameters of my tanks through electronic monitors, including temperature, salinity, Alkalinity, Magnesium, Calcium, and water levels to alert me when water is getting too low. There are a great many other options available, but these are where I've started.  

• Return pumps.  Yes, that word is intentionally plural; water filtration is so important that I make sure I have two pumps returning water, just in case one of them fails.  We'll also cover redundancy in a future post.

Space is a further consideration.  It's great that my 300 gallon tank should have between 100 and 150 gallons of capacity, but that doesn't mean that there is enough room below my tank for such a large volume.  This is something you'll need to measure for and be prepared to compromise on.

Finally, as with the entire system, any joint, any seam, any sealed item is prone to failure.  By having a sump under your tank, you've introduced several new failure points.  This is nothing to lose sleep over, but I do also have water leak detectors in all of my undertank areas just in case.  Just in case is a hugely important phrase; I encourage you to practice the same mantra.

Hopefully this has given you some ideas of what you'll need in your new hobby.  Check us out over on Facebook to leave comments if you're curious or have questions!