Big fish eats small fish.. Canibalism in my aquarium or fish nature?

Three months back I moved my Celestial Pearl Danios from the 5.5 gallons to the 20 gallons with the Black Ruby Barbs, the False JuliI Cories, the Rummynose and then I added some Harlequin Rasboras. Everything was well, I noticed Celestial Pearl Danio fry a week after moving them and I was quite happy about it. The fry grew to almost adult size, so I decided today to move the Celestial Pearl Danios back to the 5.5 gallons with their offspring. Two weeks ago I noticed a spike in ammonia so I did a major water change, and cleaned the filters. I usually get a spike every month and that's my clue that it's time to clean the filters. Every time I do a water change I count everyone, just like I used to do every morning when I first got the fish. After the water change, I had the usual 5 Celestial Pearl Danios, 1 male, and 4 females. Last week I did a 30% water change and vacuum, still 5 Celestial Pearl Danios. Today after medicating the 4 juveniles I have from the spawning with some anti-parasite medication and removing the snails, I emptied the tank and decorated it, then added the juveniles. So I went hunting for the Celestial Pearl Danios. I removed 3 females and the male. They were so hard to get, it kinda annoyed me. So kept looking for the fourth female. I was already forced to remove some of the plants to get to them in the first place. No sign of the fourth female. Sometimes they hide low in the short val's that covers 1/4 of the aquarium so I got a stick and started beating the bushes. All of a sudden I fish something.. A spine with shredded skin that had patterns of a Celestial Pearl Danio. Soon after I found a portion of a tiny fish skull.

I noticed odd behavior for a couple of days now. All the fish would come out to eat, except for the Celestial Pearl Danios. They even completely relocated to the other side of the aquarium. Usually, the Celestial Pearl Danios avoid the Black Ruby Barbs but I have never seen aggression during these three months. Plus the Danios are hard to catch and very evasive. There is no way that that healthy and young female died because of water conditions. She was a juvenile 6 months back when I got them, you can even find my post where I complain that the Danios I ordered online are "babies". So there is no way she died of old age, disease or water quality issues. The tank is cycled and the Danios look and act healthy and they are some of the hardiest fish I have seen. If there was a problem with the water, any of the Rummynose would have been the first to go. Which brings me to the most likely culprit. The Black Ruby Barb. I am not factoring out the Catfish, but they are more like "clean up crew". As the Celestial Pearl Danios grew (about 1 inch), the Black Ruby Barbs also grew (2 inches).. and even though the Danios are big enough and evasive, they are at the bottom of the food chain for an opportunistic fish. I love the Black Ruby Barbs, they are great fish, but I strongly believe one of them or more than one of them found the unsuspecting female grazing on an algae wafer or just after lights out and tore her apart. All this time they got along well and a week before I transferred the Celestial Pearl Danios back, this happens. If only I had done the move a week earlier, as planned.

On a positive note.. The Celestial Pearl Danios have been united with their fry. Even though, it could very well be their mother that was eaten. I was surprised to see the parent nature of my fish come out as soon as they saw their fry (or so I think). They spent the past 3 hours very close to each other. The male even took his fry around for schooling, I thought that was amazing as he is the biological father. They seem very protective of their young.. Ironic given that they eat their own eggs.

 

Most fish that are not strict vegetarians will go for any carcass, conspecifics included. The food chain doesn't leave anything unused. Also, it's the job of the "clean up crew" (I never got why people assume fish in that "category" wouldn't do or eat certain things.)

Sometimes a seemingly healthy fish drops dead, so one can only guess why it died and unless a ichthyologist gets a good look at it, we most often won't find out the real reason.

 

Circle of life

 

Circle of life

Exactly!

 

Yes fish die! The others just ate it. It happens can’t stop it. Probably had something to do with the ammonia spike. You should change 30% of your water weekly and clean your filter weekly or every other week. At least rinse the sponges. Ammonia will definitely hurt a fish and if it’s happening monthly it will weaken them till they die.

 

Thanks for the replies. My title was probably misleading.ead: Ever since I got back into the hobby I keep posting updates and experiences in random threads. Yes, I am obsessed with my fish, who in this forum isn't. Yes, I know they eat carcasses, which animal doesn't. I do approximately 50% weekly water changes and often over. But I am not taking the risk to clean the filter or rise the filter sponge on a weekly basis with 22 fish in a 20 gallons tank. If something goes wrong that's a massive die-off waiting to happen. So I have to be careful until I know more information to change a move... not like there is some sort of crash course aquarists manual for all the unforeseen stuff that happens in the hobby.

 

You shouldn't be getting monthly spikes in ammonia with a completely cycled tank. There is something you are doing to cause this. It may be that you are cleaning the filters too much resulting in the loss of BB. Without further information it is difficult to advise you further.

 

Thanks for the replies. My title was probably misleading.ead: Ever since I got back into the hobby I keep posting updates and experiences in random threads. Yes, I am obsessed with my fish, who in this forum isn't. Yes, I know they eat carcasses, which animal doesn't...

Many, if not most, fish do more than eat carcasses. They actively hunt anything that will fit in their mouths and eat it while it's still alive. And regarding the "which animal doesn't" part of your statement, many terrestrial animals don't.

 

With a 50% weekly water change it’s odd that your getting an ammonia spike. What type of filter are you using? When does the spike happen? Before or after cleaning the filter? How high is the spike?
Also the fish just could have died. Nothing to do with the ammonia.

 

I am using a Marineland biowheel 350 (for 70 gallons) as well as a Marineland biowheel 100 (for 20 gallons), in a 20 gallons tank. I usually get a 0.25 ppm to 0.50 ppm spike at the end of the month maybe a month and a half after I rinse the filter pads. I don't rinse the inside of the filter at all. In all the filters I have those minI clay pots for the beneficial bacteria. I always do at least 50% water changes every weekend, but I rinse the filter pads during the first maintenance week of each month, maybe the second week if I get a little busy. Once I had an unexplained 1ppm spike a couple of weeks back. In the past I had bladder snails in the 20 gallons, they disappeared for a very long time but last week I saw 4 small ones in the tank so I guess they are still around an contribute to spikes with them dying. I find some in the filter from time to time. I have about 50 bladder snails in a small bowl after I removed them from the 5.5 gallons when I moved the CPD's back in the 5.5 gallons. I don't think it is a good idea to add the snails in any tank because of the spikes they give off when they die.

Edit: Actually, I changed my mind. I will add the snails back in the 5.5 gallons as I noticed green algae clustered in tiny dots at the bottom all over the gravel. I need something that will eat it. I don't know why the 5.5 gallons has such a tough algae issue. The water was always green ever since I removed the filter when the batch of fry was born, before that the algae was on the walls of the tank. But it seems like I will add red cherry shrimp and the snails to keep the algae at bay. Perhaps I will even add some dwarf cory's to clean the gravel. Fortunately, I don't have an algae issue with the 20 gallons. I mean algae on the walls is fine but on the gravel.. It kind of irritates me.