10 + dead fish in 3 days time?

Question
We have a 55 gallon tank at work that is maintained monthly by an aquarium service.  During the last visit, we asked that our over-sized pleco be replaced with a smaller one and for some additional fish to be added.  All was well, it seemed until day 10 when we found 6 dead fish.  Including the new pleco and a new clown loach.  2 days later 6 more dead fish.  The only fish remaining are 6 tiger barbs that they added along with the pleco and clown loach.  They came for a water sample and said our ph was super low and our nitrates were off the board.  Any ideas what could cause such death?  How can water become that toxic to all species but 1?  I'm at a loss as what to think.  All of our original fish have perished.  We are left with only barbs.  And a total bill of $138.00

Answer
Hi Jennifer,

It sounds like your pH has crashed. This happens when you have low alkalinity...test your source water (which I imagine is how the aquarium gets filled, water changed out) it probably does not have high alkalinity. Alkalinity probably tests in the low to moderate range, and since its alkalinity that buffers pH, you need to do water changes weekly or at least twice monthly to replenish the alkalinity.

High nitrates are another product of lack of water changes. Since the beneficial bacteria in an aquarium convert ammonia into nitrite, and nitrite into nitrate, the nitrate has to either become plant food (live plants consume nitrate) or it has to be diluted with water changes. An aquarium that has hard water can go longer without water changes, because the high pH, alkalinity and hardness resist the natural acidification (caused by decaying food and fish waste) - however if you do not have hard water, weekly water changes are a must. The longest you can go is about two weeks without one.

You don't mention what kind of fish died except for the new pleco and one clown loach, but I suspect your tank was stocked on the full side. A tank that only receives monthly maintenance needs to be lightly stocked, and overfiltered. Not sure what your filtration is like on that tank, but when you add new fish it needs to be done slowly, so that the beneficial bacteria in the filter have time to adapt to the increased bio-load. There is only so much that they can adapt to - a 20 gallon tank can't ever hold a 10 inch oscar, all the filtration in the world can't keep that tank clean. The beneficial bacteria will always be overwhelmed by the waste, causing water quality issues.

I hope those explanations help somewhat. Your low pH and high nitrates suggest that the tank is not receiving sufficient "pollution dilution" and is becoming acidic. This is sometimes called "old tank syndrome" - Google that to find out more about it. In any case, it is up to your maintenance company to advise you on what to do next. You may need to get maintenance more than once a month, or you may need to purchase additional filtration - or you may simply need to stock this tank more lightly. Lots of brightly colored fast moving fish (such as rasboras and tetras) still look stunning in a school. It is possible to have a tank with lots of movement and color and still be understocked.

Clown loaches are delicate (prone to ich and fungus) and need sinking foods, I would not recommend them for an office type of tank. Tiger barbs are a fine choice because they are so hardy, but they will nip at long flowing fins (angelfish nipping is notorious amongst this species). The reason the tiger barbs survived is precisely because they are so hardy, able to withstand high nitrates, and they are fish that can tolerate soft water. Goldfish, by contrast, are hardy and can stand nitrate, but can't tolerate the crash in pH.

Your maintenance company ought to recommend how to solve this dilemma. They ought to understand what to do in the aftermath of a pH crash. For one thing, water changes need to be employed to reduce those nitrates. I am not sure how this works - but I would assume that they would take responsibility and do the necessary water changes and pH buffering to get you out of the red, and then advise what you need next in order to make your system viable, such as more maintenance or additional filtration. Or maybe converting to live plants.

I suggest you write Wet Web Media and present your question to them. Bob Fenner is the founder of the site and has lots of experience, having been in the aquarium maintenance business. I have a feeling he will say this company is shady...a freshwater tank is really not difficult to maintain compared to a marine reef tank, which is probably what they are getting called on to install most often. If they haven't suggested a "plan of attack" for getting your aquarium balanced again, I would say, ditch this company and look for another!

Here's the link to instructions on writing Wet Web Media:
http://www.wetwebmedia.com/WWMAdminSubWebIndex/question_page.htm

I hope that helps, take care!
Nicole