So, without further preamble, here are the Top 10 Disasters You'll Face in the aquarium Hobby:

#1 - You Are Going to Spend Way More Money Than You Anticipated!  

The big box stores sell entry level aquariums for anywhere between $1 - $3 per gallon, depending on the size, type, brand, and what type of deals are going on.  A Local Fish Store (LFS) will charge  higher prices for entry level aquariums starting at about $3 a gallon, again based on the same factors, though you're going to get a better product at that LFS.   In both cases, this is the tiniest fraction of what you'll need to buy in order to host living animals.  You'll need filtration, water chemistry modifiers and test kits, a substrate like rock or sand,  a pump, a heater, water agitators, and the fish themselves.  If you're going with a saltwater tank, you'll have a few more requirements, including a source of salinity, live rock, a cleanup crew, and more test kits.  

I'm often asked how much my saltwater reef tanks cost - which is somewhat a misnomer question because the tank is just the container; but assuming the question being asked is "How much would it cost me to duplicate what you have here", the answer is typically between $30 - $40 per gallon of tank.  The math on a 100 gallon tank quickly becomes quite daunting, and although there are some economies of scale, my two 300 gallon tanks alone are marriage-breakers.  Freshwater tanks are significantly cheaper with costs closer to $10 / gallon because the water chemistry is easier; saltwater tanks that are Fish Only With Live Rock (FOWLR) are more complex than freshwater, but easier than reef tanks because the fish do not require as pristine of water as corals do to survive, which means less equipment and easier chemistry tolerances.

#2 - Your New Fish and Coral Friends Are Trying Their Hardest to Die on Your Watch!

I still have flashbacks of my early years in aquariums: coming out every morning to see if anyone was floating.  It was mindbogglingly frustrating that I kept trying so hard and yet getting it wrong and I'd spend the rest of the day full of remorse over not being a better pet owner.  What caused the death is sometimes easy to learn, and sometimes never has an answer.  But this is the reality of aquariums: you're creating a microcosm that is a living biome, out of your control, and often you can only identify an issue AFTER something is floating , withering, or turning brown and then white.  Once an established biome has matured in your tank, thankfully a lot of these surprises go away.  The real tragedy here is with inexperienced hobbyists who want to move faster and introduce more without understanding the balance required to proceed.  This invariably ends at Davey Jones' Locker.

#3 - Fish are Suicidal

In addition to all the ways your inexperience can kill your fish, your finned friends are determined to starve themselves,  bully or be bullied by their roommates, and are prone to jump out of the tank to their non-watery deaths!  I remember thinking that one of the tangs I bought was really good at playing hide and seek - and was very good at being deep in my aquascape.  Then one day while I'm under my tank working on a monitoring system I found a desiccated fish laying in my overflow tray.  The fish had jumped out of my tank through a gap in the back of my topper.   Acclimating your new fish to a tank full of established residents often leads to bullying and death.  Bringing a new fish home can also bring all types of new unwanted pests that can wipe out an entire tank, and if you're not quarantining your new fish (and corals!) before adding them to your existing community, you're playing a game of Russian Roulette.  

#4 - Equipment Failures are Guaranteed

All that expensive equipment you bought to support your tankmates is actually hobby grade equipment.  When you chose the Jebao return pump over the Abyzz pump because the former was $200 and the latter was $2,000 - that decision came with a Mean Time Between Failure that you will have to account for in your maintenance planning.  Heaters are notirious for failing and often in the on state, which is a horrible way to make fish stew.  (Heaters are so notorious, I recommend yearly change outs. ) The mechanical pieces turn on hundreds of times per day and operate 24 hours a day in perpetuity - right up until they don't.  While there are no guarantees, by spending extra on better equipment, you're removing a classic failure point.  

#5 - Water Chemistry is Hard!

We think very little about the air we breathe inside our homes.  It's there, it's available, it's good.  But in an airtight container, we know that things will get really bad if we don't open a vent.  Your tank inhabitants have the same constraints and you, the owner, are the only one who can make sure they're breathing good air.  This is so important that I tell my clients: " We are not in the business of maintaining fish or corals - we're in the business of maintaining water quality; the rest takes care of itself."  

#6 - Just Because You're Busy Isn't an Excuse

The distance you can travel from home is dependent on your ability to get fish care.  While a healthy tank can easily support its inhabitants for a few days on occasion without you, this is very risky.  In addition to all the things that can go wrong with equipment and water chemistry, there are also issues with stress, pests, diseases, etc.  Owning an aquarium is a daily commitment. 

#7 - You'll Get Pennies on the Dollar in Resale

I see it on Facebook Marketplace and eBay all the time - someone getting out of the hobby and trying to get a price they think is fair in return for their original investment when they decide to sell.  You will find that while you might spend $30 to $40 per gallon to get into the hobby (new everything), getting out will return you anywhere from $3 to $10 / gallon in return.  The secondary market is saturated with people trying to sell equipment that the buyer has no idea of how it was cared for, what condition or issues the equipment might have, etc.  It's brutal how little people get for "Complete setups" and if you find yourself needing to get out of the hobby, it's usually smarter to part out the tank.  Even then, you're still in the same range, though usually on the higher end.

#8 - Your First Tank is Usually the Wrong Tank

You just don't know what you don't know.  You're looking at things in a store in a pristine environment; you've got professionals who've set up the things you're looking at.  You've got a salesperson talking you into this or that - or the store's sale sways your decision.  If you decide that you love the hobby and you want to stay in the hobby, your first tank is never the tank you wanted.  You'll want bigger, additional, more complex, more automated, more everything.  This is the nature of the hobby, and the entire industry supporting the hobby is built to keep you focused on what you want, not what you have.

#9 - You're Going to Flood Your House, Probably Multiple Times

Water is the most common chemical compound that you're going to be dealing with in this hobby, from water changes (a major contributor to a healthy tank) or from top-off water (adding freshwater that keeps the ratio of NaCl : H2O constant is important because of natural evaporation), and the bigger your tank the more water you'll be moving.  In order to get water for freshwater tanks, it's a simple matter of adding a  chlorine bonding agent that keeps your fish from being poisoned.  In order to provide water for a saltwater tank it's important to start with pure water that has been put through a reverse osmosis (RO) filter and has been de-ionized (DI) via a chemical reaction; next an aquarium appropriate salt is added, mixed for 2 to 24 hours, heated, and ready to be added to your tank.  The process is open to any number of opportunities for disaster, from forgetting that you're transferring water (it's time consuming and invariably you'll want to do something else while it's moving... and then forgetful you will finally remember when you hear splashing water and wonder why), to leaking tanks, ruptured hoses, hose connections that come loose, a dog under foot while moving a bucket, etc.  There's a saying in the hobby: Get a tank so you can never be dry again.

#10 - Absolutely Nothing Good Happens Fast

If you're looking for a quick payoff, say from adorable puppy to reliable guard dog within a year, then reefing isn't your hobby.  Most of us start with small fish and small pieces of corals called Frags glued to oversized ceramic thumbtacks, and then wait for them to grow up into a showpiece tank.  That's a long process, and the bigger your tank, the slower it will happen.  There are no shortcuts short of buying fully matured livestock, and that's all but unobtainable for all but the wealthy.    For example, while you can buy a Walt Disney Acropora frag for $100, a fully grown colony would be thousands of dollars - if you can even find someone willing to sell.  The mature saltwater mixed reef is living art and worth the time and energy, but getting to that level of maturity is a multi-year journey.