Disappearing Danios

[SandstoneSturgeon]

I have a very unusual mystery that is beyond my skill level. I have a 260L tank with an API Filstar canister filter. Tank has been operational for about a year now and was originally stocked as follows:

4 Giant Danios
9 Longfin Danios
6 Black Neons
9 Glowlight tetras
5 Emerald Corys

Plants:

Water Wisteria
Java Ferns Argentine Swords

I noticed 6 months ago that my longfin Danios were dwindling in numbers, from 9 down to 1! I would search the entire tank for bodies and never found any. Nothing ever turned up in the filter and I have a tight fitting lid. Last weekend I did a rescape (pics coming ) and completely changed out the gravel for some CaribSea complete. Never found even one dead Danio. Are they getting eaten by the giants? It seems unlikely that 8 of my 9 longfins would be eaten but the much smaller and much slower swimming tetras are all still present and well. Can someone tell me where these little Houdinis have vanished to?

Thanks for your help
James A

 

[SandstoneSturgeon]

Could the Cory's have eaten the bodies before I had a chance to find them every time? I wouldn't even know what was killing them in that case.

 

[PARAGUAY]

Is it a opened top tank?Danios notorious jumpers ? I have very old Black Widow that as just passed away have removed bog wood etc to try and find unsuccessfuly just now,so am doing daily water changes and increasing filter maintainence at moment

 

[Aqua360]

probably eaten fast, those smaller fish don't tend to leave much residuals after they get eaten at the bottom

 

[Tim Harrison]

Probably eaten and/or jumped. Do you have a cat? If so it'll probably hoover up any jumpers as well, thus leaving no trace.

 

[SandstoneSturgeon]

Tank has a tight fitting lid because of the giant Danios and no cat (landlord won't allow ) so it looks like they died of something. All the other fish look very healthy and well so I'm wondering now if there are any diseases that would be specific to only them? They all appeared very healthy up to the point were they went missing but I guess that doesn't always mean they are well. Water parameters all seem good so the only other thing is maybe the hard alkaline water? All the Midwestern US has very hard water that is fairly alkaline. I use Sphagnum Moss in my filter to try and make the water a little more acidic and temporarily soften it up.

 

[alto]

Are they getting eaten by the giants? It seems unlikely that 8 of my 9 longfins would be eaten but the much smaller and much slower swimming tetras are all still present and well.

I'd expect aggression to be focused on conspecifics - in this instance the longfin danios - so not really surprising that the neons & tetras are "ignored" (likely they don't give the same "feedback" re behavior as other danio species) ... you don't mention tank size but it's generally recommended to keep 'Giant Danio' in groups of at least 10
- though if yours are peaceful, then perhaps just maintain the existing group & avoid adding other danio species, especially long fin variants which are definitely not as robust as the "wild type" ( + those fins often attract attention from other fish).
While the Giant's may not have eaten the smaller longfin danios, they may have stressed them, making them much more susceptible to "disease" - not something that will spread through your tank but the sort of chronic condition where fish just don't thrive & numbers gradually decrease, until some months later few or none of the original group remains.

It's very usual to not find a body, often fish will go into "hiding" when unwell, deterioration is very quick with fish (re specific biology) & a fish that dies late evening will often be "gone" by morning - other fish are generally rather quick to find this tasty snack

I'm wondering now if there are any diseases that would be specific to only them?

This also has merit - Matt mentions this in his Seriously Fish article, also read through notes at the end of article ... it seems likely that you would've noticed some symptoms though

Unfortunately it has been mass-produced in huge numbers for many years, thus inbreeding has occurred on a large scale and quality stock can be hard to find.

Alongside morphological deformities common ailments include ‘neon tetra disease’ (NTD), caused by the microsporidian parasite Pleistophora hyphessobryconis and basically untreatable once it reaches an advanced stage.

Symptoms include restlessness, increased respiration, white patches on the skin, and an individual separating itself from the group. The spores (larval stage) of the parasite enter the fish through the mouth and burrow through the walls of the gut before settling in the muscles. There they produce cysts which cause rapid degeneration of the muscle tissue and development of the characteristic white patches.

Once the disease takes hold mortality rates can rapidly become very high. No medication is available, and the only way to ‘treat’ the tank is to physically remove any fish displaying symptoms.

Similar symptoms are caused by a severe form of the pathogenic bacterium Flavobacterium columnare, and this is sometimes mistaken for NTD (Michel, Messiaen, and Bernardet, 2002).

Another common problem is microsporidiosis, caused by the parasite Pseudoloma neurofilia (Matthews et al. 2001), with external symptoms including chronic emaciation and curvature of the spine having led to the vernacular term ‘skinny disease’. Its life-cycle is similar to that of F. columnare in that it is ingested orally, but the main site of infection is the central nervous system, and it can be tricky to diagnose since infestations have been recorded in seemingly healthy individuals.

 

[SandstoneSturgeon]

Sorry about not posting the dimensions.Tank is 121cm long, 30cm wide, and 76cm tall. I had been concerned about putting too many giants in because of the size, do you think I could add more giants? I really enjoy their energy and hyperactivity.

Interesting that the article that was posted by alto (thanks for sharing that, btw. I've favorited it and will look to sign up ) mentions that, zebra Danios do not favor strong currents in the wild.Their counter parts, the giant Danios, love to swim into the strong current that I have set up with my canister filter. The upper half of the tank has a good swift flow for the giants and the lower half is protected with lava rocks to provide calmer waters for the tetras. Danios like open spaces to swim and mine always hung out at the top. I,wonder if that strong current stressed them?

 

[alto]

Seriously Fish is an outstanding site (referenced articles!)

Devario malabaricus your "giants" may be this species (I suspect the max standard length given is an error, 15cm is more usual SL listed) - if you put "danio" into the SF search engine, lots of interesting species

Danios like open spaces to swim and mine always hung out at the top

I wonder then ... Zebra danios are renowned for their adaptability, it's unlike them to not integrate into a tank
- either disease or aggression from the Giants would account for this behaviour: once the subordinate fish have been trounced, often all it takes is a fin flip from the Boss to keep them in place, so it's not unusual to observe very little to no chasing or signs of aggression

How many Giants you can keep depends on your filtration & water change routine - how big are your present group? not overfeeding is a significant factor in bioload.
If you net search, you can find several discussions on keeping this fish.

 

[SandstoneSturgeon]

My four are fully grown. The smallest is about 4.5 inches and the alpha boss is probably a little over 5 inches. All discussions that I read suggest a group of at least 8 for peaceful results. Might be time to add a few more of the giants.