Any Idea What Killed My Fish?

[dwiles]

Hi guys wondering if you can help me I've had a couple of fish deaths in the last few days.
 
The tank is nearly 3 months old.
 
90 litre JBL nano - converted marine to freshwater. Filter powered by a 1000LPH pump, running with floss, ceramic rings and purigen.
 
The tank contained
 8 cardinals
 4 Leopard corydoras
 1 L201 Snowflake pleco
 2 Lyretail Killifish (pair)
 
The killifish I added on saturday, the rest of the fish have been in there for a while.
 
Killis seemed to be doing fine, didn't see them being bothered by other fish, swimming happily. I went out that night and didnt come home til the next evening, when I returned I couldn't see the male so I took the tank cover off to see if he'd jumped into the back of the tank. I then spotted him jumping about but I didn't see if he was in the main tank or back compartment, he was in the main tank at this point, kept trying to jump out but i stopped him, he was then just floating about on his side as if he was dead, then would occasionally burst forward and start spiralling again, like he was on his was out.  
 
I left him in the tank as I thought he may be in shock then immediately did a water change. Tested the water and results were
PH = 7.5
Ammonia = 0.1 mg/l
Nitrite = 0.1 mg/l
Nitrate = 20 mg/l
 
Parameters are usually lower than this, but not greatly. I fed the fish.
 
Yesterday - No sign of the male killifish - couldn't find the body anywhere - assume he is dead. didn't feed the tank, to perhaps stabilize parameters
 
Today - I am one cardinal short and haven't seen it anywhere, assume this is also dead. Fed the tank bloodworm and did a small water change. All the fish look healthy and active (as they usually do). Parameters are now:
PH - 7
Ammonia - 0.1 mg/l
Nitrite - 0.05 mg/l
Nitrate - 20 mg/l 
 
So my question is; would anybody be able to tell me why the two fish may have died? And would the two deaths be related? My two main concerns would be 1. the filtration isn't adequate enough and 2. the pleco may have attacked both the fish at night? 
 
Any help and advice is greatly appreciated thank you

 

[RainboWBacoN420]

How developed is that pleco? It's not uncommon for plecos to develop a taste for meat as they get older. Your Killifish could have been jumpy from the ammonia and nitrite, but then again, Killifish are simply known for just being jumpers. If the Killifish is still alive, he could of also have eaten your cardinal. I just witnessed this yesterday at my job, we have a tank full of Killifish and cardinals and at least three times, I saw a Killifish wish an entire cardinal in it's mouth. These cardinals were just added though, so they were most likely dying or very weak, the healthy ones seemed fine, but since you have ammonia and nitrite, that could be the reason for a weak cardinal. 
 
But my guess is the pleco...which could also be very well responsible for your parameters becoming out of check. Plecos, no matter what size always give off a large bio-load, with how small your tank is along with the other fish you have in there, I'd say your tank is a bit overstocked. 
 
What do you feed your fish, how often and how much? 
 
Do you know what your General Hardness is?
 
And lastly, what method do you use to introduce fish into the aquarium and what was the last fish you put in there?

 

[dwiles]

How developed is that pleco? It's not uncommon for plecos to develop a taste for meat as they get older. Your Killifish could have been jumpy from the ammonia and nitrite, but then again, Killifish are simply known for just being jumpers. If the Killifish is still alive, he could of also have eaten your cardinal. I just witnessed this yesterday at my job, we have a tank full of Killifish and cardinals and at least three times, I saw a Killifish wish an entire cardinal in it's mouth. These cardinals were just added though, so they were most likely dying or very weak, the healthy ones seemed fine, but since you have ammonia and nitrite, that could be the reason for a weak cardinal. 
 
But my guess is the pleco...which could also be very well responsible for your parameters becoming out of check. Plecos, no matter what size always give off a large bio-load, with how small your tank is along with the other fish you have in there, I'd say your tank is a bit overstocked. 
 
What do you feed your fish, how often and how much? 
 
Do you know what your General Hardness is?
 
And lastly, what method do you use to introduce fish into the aquarium and what was the last fish you put in there?

 
 
Thanks for the reply! The pleco is around 2.5 inches long, and has been in the tank for around 1 month. This is the first instance of any fish death in the tank.
 
The Female killi is still alive, the male hasn't been seen since sunday and I would say he's therefore dead - he looked very ill when he was last seen. The killifish are smaller than the cardinals, and I didn't see any aggressive behaviour between either fish.
 
I feed them on a mixed diet of flake, discuss pellets, mysis and bloodworm (different foods different days). Some days I do not feed at all as I was told this can be helpful.
 
I do not test for general hardness unfortunately.
 
The last fish I put in were the killifish pair which were added on saturday, all the other fish have been in there for over a month. I float them in the bag and gradually add water over the course of about an hour.
 
Would it be wise to do another water change now? 

 

[RainboWBacoN420]

If you're still picking up readings on ammonia and nitrite, then yes, do a 30-50% water change. 
Your Killis are smaller than the cardinals? They must be some big cardinals. 
 
Invest in a GH test solution, they're a wise investment for the hobby.
 
Your pleco probably ate your Killi, and cardinal, they could of have both died before being eaten, but as for the whereabouts of their bodies... most likely in the tummy of your pleco. 
 
The way you introduce your fish is fine, however there are better methods and recommend this to everyone. Use the drip method acclimation, it's far less stressful on the fish and usually yields success all the time. It's also the most professional method to take. 
 
One possibly as for the death of your Killi and cardinal, is that the Killi was sick and gave the illness to the cardinal, how did the Killi look ill before he disappeared? 

 

[dwiles]

If you're still picking up readings on ammonia and nitrite, then yes, do a 30-50% water change. 
Your Killis are smaller than the cardinals? They must be some big cardinals. 
 
Invest in a GH test solution, they're a wise investment for the hobby.
 
Your pleco probably ate your Killi, and cardinal, they could of have both died before being eaten, but as for the whereabouts of their bodies... most likely in the tummy of your pleco. 
 
The way you introduce your fish is fine, however there are better methods and recommend this to everyone. Use the drip method acclimation, it's far less stressful on the fish and usually yields success all the time. It's also the most professional method to take. 
 
One possibly as for the death of your Killi and cardinal, is that the Killi was sick and gave the illness to the cardinal, how did the Killi look ill before he disappeared? 

 
Thanks I shall do that now.
 
yeah I would say the killis are about an inch long, the cardinals just over. Thanks for your help, would you say killifish are particularly sensitive to slightly poor water quality? I just need to find a store which will take the pleco off my hands, hopefully this weekend!
 
I will look into drip acclimatisation and do this for any future fish I add.
 
The male killifish just seemed as if he was almost dead, he kept trying to jump out of the water, then would drift away not moving, then suddenly wriggle again etc. Complete loss of balance.

 

[RainboWBacoN420]

Killifish are said to be very hardy fish. Of course, it could vary with different types of Killifish, I'm not sure of this, though.