My harlequin rasboras seem prone to suicide by jumping out of the tank. I have a hypothesis ... but also want to check with the community here for any other ideas.
This is my first serious high tech planted tank, and my first rimless and cover-less tank. Its a 18"x18"x36" (45 gallons or so) rimless tank, CO2 injection by conventional diffuser, filtering with Oase Biomaster 350 using a lily pipe outflow and a skimmer inflow. Substrate is UNS Controsoil aquasoil capped with Seachem Fluorite. Lighting is two Kessil A350WE running lower than maximum brightness with a fairly short photoperiod Water parameters are
kH 4-5 degrees
GH ~12 degrees
pH about 6.8 before CO2 comes on, drops to about 5.9 with CO2 Drop checker eventually gets to yellowish-green by the end of the day with pH at 5.9
-- tap water here has pH 7.4-7.6; in this tank with the aquasoil substrate it lowers to about 6.8
The tank took about a month to cycle fully before I added any livestock, and I've gotten through the first two months of the tanks life with only minor algae issues. So far the plant growth is pretty acceptable by my standards.
the fauna includes a (dwindling) school of Rasbora Het, a (growing) school of white cloud minnows, a handful of otos and Amano shrimp and a few snails. The white clouds and otos went in first, then the harlequins and amano shrimp a couple weeks later. The white clouds spawned and the fry are growing up in a breeder net hanging in the tank.
Only the harlequin rasboras are jumping out of the tank- I've lost 3 over the past few weeks since I added them. None of the minnows have leapt to their deaths.
The pattern I notice is that the rasboras jump out some time in the early morning. In both cases of jumping (one fish the first time, two fish the second time), there were no jumpers at or after midnight but at 8am there was a dead fish or two on the floor. The morning after the double suicide, I also found one rasbora limply floating and thought it was dead but it swam away when I disturbed it and later in the day seemed perfectly normal.
My current hypothesis is that the CO2 running before the lights turn on got too high without enough dissolved oxygen. I had the CO2 starting up 2.5hrs before the lights came on. The pH doesn't go below 5.9 in the morning but the tank is heavily planted. Even with lots of surface agitation and the skimmer keeping the surface free of scum, I wonder if the plant respiration is creating a low oxygen environment that the fish are trying to escape.
The otos and white clouds and shrimp don't seem to have any problem and the pH drop is less than what many on this forum report is possible while maintaining healthy fish. I don't see the rest of the fish gasping at the surface of the water.
For now I've shortened the time the CO2 runs before the lights come on to just 1 hour and I'll see if that stops the suicides. But in the interest of saving more fish before I figure it out, I'm happy for any other recommendations.
Are harlequin rasboras known to be jumpers? I hesitated to add a cover to the beautiful rimless tank which also would make the CO2 management more challenging. But neither do I want to be killing fish.
This is my first serious high tech planted tank, and my first rimless and cover-less tank. Its a 18"x18"x36" (45 gallons or so) rimless tank, CO2 injection by conventional diffuser, filtering with Oase Biomaster 350 using a lily pipe outflow and a skimmer inflow. Substrate is UNS Controsoil aquasoil capped with Seachem Fluorite. Lighting is two Kessil A350WE running lower than maximum brightness with a fairly short photoperiod Water parameters are
kH 4-5 degrees
GH ~12 degrees
pH about 6.8 before CO2 comes on, drops to about 5.9 with CO2 Drop checker eventually gets to yellowish-green by the end of the day with pH at 5.9
-- tap water here has pH 7.4-7.6; in this tank with the aquasoil substrate it lowers to about 6.8
The tank took about a month to cycle fully before I added any livestock, and I've gotten through the first two months of the tanks life with only minor algae issues. So far the plant growth is pretty acceptable by my standards.
the fauna includes a (dwindling) school of Rasbora Het, a (growing) school of white cloud minnows, a handful of otos and Amano shrimp and a few snails. The white clouds and otos went in first, then the harlequins and amano shrimp a couple weeks later. The white clouds spawned and the fry are growing up in a breeder net hanging in the tank.
Only the harlequin rasboras are jumping out of the tank- I've lost 3 over the past few weeks since I added them. None of the minnows have leapt to their deaths.
The pattern I notice is that the rasboras jump out some time in the early morning. In both cases of jumping (one fish the first time, two fish the second time), there were no jumpers at or after midnight but at 8am there was a dead fish or two on the floor. The morning after the double suicide, I also found one rasbora limply floating and thought it was dead but it swam away when I disturbed it and later in the day seemed perfectly normal.
My current hypothesis is that the CO2 running before the lights turn on got too high without enough dissolved oxygen. I had the CO2 starting up 2.5hrs before the lights came on. The pH doesn't go below 5.9 in the morning but the tank is heavily planted. Even with lots of surface agitation and the skimmer keeping the surface free of scum, I wonder if the plant respiration is creating a low oxygen environment that the fish are trying to escape.
The otos and white clouds and shrimp don't seem to have any problem and the pH drop is less than what many on this forum report is possible while maintaining healthy fish. I don't see the rest of the fish gasping at the surface of the water.
For now I've shortened the time the CO2 runs before the lights come on to just 1 hour and I'll see if that stops the suicides. But in the interest of saving more fish before I figure it out, I'm happy for any other recommendations.
Are harlequin rasboras known to be jumpers? I hesitated to add a cover to the beautiful rimless tank which also would make the CO2 management more challenging. But neither do I want to be killing fish.
Last edited:
