Tank Crashes / Causes and Avoidance

The absolute worst common aquarium problem is the dreaded tank crash!  There is no worse experience as an aquarists than to have your tankful of fish that you’ve worked so hard to make a good home for wind up dead.  There are a number of reasons why this can happen but with some precautions the danger of a tank crash can nearly be eliminated.

The primary cause of a tank crash is lack of oxygen in the aquarium.  Without proper aeration and water circulation the aquarium can literally become a death trap for the fish in it.  This can literally occur overnight in many cases.  This is why you can not rely on any single source of water circulation and aeration in the tank and really, three separate sources should be used at all times in order to minimize the chances of a tank crash due to lack of oxygen.

A hang on back or canister filter is a common primary source of water circulation in many aquariums.  But what happens when they gradually slow down due to becoming dirty or their impeller becomes clogged?  If this is the sole source of aeration in your aquarium then a tank crash becomes imminent.

A powerhead designed primarily to provide water circulation is a great choice for a backup source of aeration.  However, if you do a water change and lower the powerhead and forget to move it back up then it becomes useless as a source of aeration.  Water movement in the middle of the tank will not aerate the water.  Water motion and movement should be visible on the surface of the tank at all time.

In saltwater aquariums, a skimmer will provide a great third source of additional aeration.  In a freshwater (or saltwater) aquarium an inexpensive air pump connected to a bubbler will provide this function.

Three sources of water movement and/or aeration will greatly reduce the chance of a tank crash due to lack of oxygen in the tank.  Care should be taken to ensure all are working properly at all times.  In a small tank with just a few inexpensive fish, you can get by with two but for larger systems we would consider three the minimum.  A battery powered aerator is important to keep handy in the event of a power outage that lasts more than just a couple of hours.  A quick visual check of all systems before you go to bed at night can help prevent a disaster when you wake up the next morning.  Many fish sleep on the bottom of the tank where the oxygen will be the lowest in the event of a failure and if the oxygen dips too low they might never wake up.

Tank Crash due to other water parameters

Low Oxygen isn’t the only possible cause of a tank crash.  A dramatic temperature fluctuation or pH fluctuation is also possible.  Typically these can be avoided with regular monitoring of the aquarium system and are unlikely to occur as fast as a tank crash due to a lack of aeration.

Tank Crashes can occur due to temperature fluctuations.  If the temperature rises over 82 or dips below 68 degrees then loss of life can occur for many fish species.  This can happen in a hot house without air conditioning.  It can also occur due to a faulty tank heater.  An inexpensive thermometer that is checked regularly will help you gauge the effectiveness of the tank heater as well as the effect of the temperature of your home on your tank.  Also, learn how the water feels when it is at the correct temperature.  When you feed or do routine maintenance you will be able to tell if the temperature of the water needs attention.  When you first buy a new water heater make sure you check it initially as there are occasional reports of an aquarium heaters thermostat not working properly and over heating.  Also, keep in mind that they can quit heating and do periodically need to be replaced.

An ammonia spike can also cause a tank crash.  This is of course a great danger when initially cycling the aquarium but can occur after the initial cycling.  If you understand why it can occur after the initial cycle then you can be prepared and avoid it.  In lightly and even moderately stocked tanks there will typically be enough bacteria in the aquarium systems to handle occasional ammonia fluctuations even without any other biofiltration.  Even just a little biofiltration such as a biowheel will be more than enough to handle even significant ammonia fluctuations in these tanks.  Ammonia fluctuations can occur when adding fish to the system or when a fish dies and decomposes without you realizing it.  However, if you greatly increase the number of fish in the system at once or several fish die at once and there is no biofiltration in the system then it is possible to have an ammonia spike which can subsequently crash the entire tank.  It is a good idea to have ammonia test strips handy as well as ammonia neutralizing water conditioner around at all times specifically so you can monitor and neutralize any potential dangerous situations.  In heavily stocked aquariums this possibility is increased and you should be on the look out for any dead fish and try to remove them from the system as soon as possible.  Also, in heavily stocked systems, additional biofiltration such as a biological filter pad or biowheel is a necessary component.  However, heavy biofiltration is still not necessary and will convert all organics in the water column into nitrates very rapidly.  This will reduce the amount of organics available to other microfauna and can compete with denitrification systems and create a sterile system with chronically high nitrates.

A tank crash is also possible due to a dramatic pH fluctuation.  This will not happen overnight but over a few weeks.  In neutral pH freshwater tanks a quick test strip test is adequate to make sure the tank pH is not dropping or rising to far too fast.  Generally a range between 6.5 and 7.5 will be fine for most freshwater fish.  For saltwater or high pH freshwater tanks the pH is a little harder to monitor with test strips and we recommend using a high range pH test kit and monitoring the tank weekly.  The pH in a high pH tank will naturally drop over time (unlike in a neutral pH tank) and you will sometimes need to add baking soda or other buffering additives to maintain the pH.   In saltwater aquariums the API high range pH kit is an essential tool to have.  At 8.0 the test will appear a light brown and at 8.4 the test will appear to be a purple.  You will want to monitor weekly … when it is a light brown you will add buffer until the test appears a light purple.  Otherwise a quick and dramatic fluctuation will not occur and you can easily prevent a tank crash caused by inadequate pH.

There are other factors that will not crash a tank but can gradually kill organisms in the tank and reduce the overall quality of the aquarium system.

For coral and planted tanks proper lighting is necessary.  In coral tanks, maintaining proper calcium levels and other trace element levels in the tank is necessary.  In planted tanks, calcium is not an issue but having an appropriate substrate or occasional use of trace element fertilizer is important.  Failure to do so can result in the death of plants and corals.  Some other invertebrates will also require a proper balance of trace elements.  The use of quality trace element supplements and water changes, occasional water changes when denitrification is used, will ensure that the water quality remains high for all organisms.

Ideally, an aquarium system will be as close to a healthy natural water ecosystem as possible.  A healthy natural system will have a healthy microfauna system with rotifers and copepods and even some organic molecules, negligible nitrates, plenty of oxygen, low phosphates, good water movement, and the proper levels of all trace elements.  pH, Salinity, temperature, overall hardness, and to some extent the type of trace elements needed (such as calcium) are really the only species specific variables.  Of those, oxygen absolutely is the most important factor.  Salinity once established is pretty easy to maintain.  pH and hardness is important but fluctuates slowly and in neutral tanks fluctuates very slowly if at all.  Temperature is important but usually fluctuates little or slowly (except when a heater malfunctions) and fish can handle a fair range.  Nitrates are an issue, especially without denitrification, and are a major focus of aquarium systems … but typically will not cause a tank crash but instead just cause chronic problems and increase chance of fish disease, algae issues, and other long term problems in the aquarium.  Ammonia is a huge danger initially but after the initial cycle can generally be addressed without much problem.   Phosphates, calcium levels, and other trace elements are not typically a major problem but do need to be considered over the long term in order to maintain a well balances and healthy natural aquatic ecosystem.  Microfauna are definitely beneficial to maintain, especially in reef tanks or aquariums with any filter feeders … however some systems such as fish only systems can even function well without them at all.