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I've been using wha we call shell rock. It is shells and fossils. it was working well untill the end of the summer. the tank water got very acid (purple in test kit) and dead fish. Is this a result of the growing medium or just not enough grow bed to cleanse the water?

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What kind of test kit?  The API kit, purple is actually a very high pH which is the opposite of acid.


To be able to answer the rest of your question, we need to know what the other tests said as well about ammonia, nitrite, and nitrate.


Also how much grow bed did you have?  How much fish tank?  What kind of pumping?  And how many fish/what size?


See actual shells I would expect to cause a high pH but fossils are different and I'm not sure what sort of pH effect they would have if any.  If your pH was in a good range around 7 while things were working well and then if the pH really did drop below say 6 or 5, you could well have had a pH crash.  In this case your system ran our of alkalinity and buffer and the pH dropped so low that the bacteria quit working.  In that case the ammonia would have spiked and this can definitely kill fish if not caught in time.  This is why we recommend keeping an eye on the pH regularly.  There are many materials that can be added in small amounts to keep the pH from dropping too low, you just need to make sure not to add too much which could raise the pH too high too fast.  Most people try to keep their pH between 6.8-7.0.


So please confirm what the number was for your pH.

The fossils and shells are dug up at the same location. We normally use it to cover dirt roads. It's less expensive than gravel and has the same consistency as gravel. I was interested to see how it would affect the PH, being as how it comes from Marine life. I have heard of people putting oyster shells at the growbed line to help (as they say) stablize the PH. But mine is all in the bed. I'm flushing the medium now to try it again. The growbed is only 5'x7'x6", 35gal tank, 10- 6" catfish, flood bed every 2hr, any suggestions?

I would love to move to a raft system but am unsure of the mechanics. If the bacteria is on the gravel to change the ammonia to nitrites and nitrates, what transforms the ammonia in a raft system with no gravel?

I would keep the gravel bed and then you could add a raft bed.


Keep in mind that the shells people add to a system tend to keep pH up and if you put in too many, it can keep the pH too high.  Believe me, I've got a system with about 45% of the media being sea shells, pH stays high so I have to add chelated iron regularly.  If you use shells to buffer pH in a system, I recommend putting them in  a mesh bag or stocking that you can hang in the fish tank and pull out as needed.  You actually want your pH to be around 6.8-7.  High pH numbers are actually alkali while low pH numbers are actually more acidic.


So what test kit are you using that you say your pH was purple but that means acid?  As I said, the API high range pH kit, purple is actually a really high pH and alkali not acid.  I have had a situation where my pH was driven that high by algae and if there was any ammonia in the system when the pH went so high, that could kill fish too since ammonia is far more toxic the higher the pH and the higher the temperature.


10 catfish in a 35 gallon tank is rather a lot of fish for such a small tank.  (Granted, I grow channel catfish and they get big so I usually give each fish closer to 10 gallons worth of fish tank each.)  Now your grow bed is probably big enough to filter for those ten fish but if you were only flooding it once every two hours, the ammonia and such could have been building up to dangerous levels for your fish in such a small fish tank.  Do you also have a sump on this system.  Seems that the water would fluctuate quite a bit in such a small fish tank.  It is generally recommended that the fish tank volume be pumped at least once each hour and in my systems I often move the water around much more than that.  For my 300 gallon fish tank I probably pump 600 gallons an hour.


Any supplemental aeration or pumping?  A 35 gallon fish tank with ten fish in it going 2 hours between water pumping could quickly run out of dissolved oxygen for the fish if there is no supplemental aeration especially when the weather is warm.  Warmer water holds less dissolved oxygen and therefore will be used up faster.



Wow, thanks, this is the info i've been looking for and unable to find. I'll move the water more frequently, I've changed to a 65 gal tank. I've buried half of it to keep it cooler in the summer. I do have air stones in it. and the water did fluctuate quite a bit before but don't think it'll be a problem now.

I am running water through 40' of 4" pvc with the plants sitting in net pots in the top. It is on the 35 gal drum and has 10 gold fish. I did put a handfull of this shell rock at the head of both tubes. The water runs for 15min every 1 1/2 hrs.

I have rafts in small tanks that run off worm castings. The plants are small but have flowers (tomatos, cucs, peppers, basil etc).

My test kit is only a swimming pool kit.

Thanks again,

ah.  An API freshwater master test kit would probably help you to better know what is going on.

Anyway, goldfish will probably do well to keep your system going if you feed them well.  I don't personally recommend growing catfish in small tanks, I've never had anything but really small fingerlings do well in anything smaller than 300 gallons.

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