Help with drop in oxygen level with Oase canister filter

Hey folks!

I have an oxygenation mystery on my hands which I *think* is related to my filter, but I'm not certain, and would love some help thinking this through!

First some context:

• Age: 1 year
• Ammonia / Nitrates / Nitrites: 0 / 0 / 10 - 20
• Tank: 16 gallon cube nano tank
• Plants: Heavily planted with both root feeders and epiphytes
• Livestock: 2x german rams, 2x peacock gudgeons, 8x emperor tetras, 1x dwarf pleco (I know it's a little overstocked, but I do weekly 25% WCs and have a pretty overpowered filter for the tank size)
• Filtration: Oase Filtosmart Thermo 200, using glass lily pipes. Temperature set at 80 degrees. Stock filter media plus some filter floss and purigen.
• CO2 injection: 1 bubble/second

So here's the issue: About two months ago, I noticed fish hanging out at the surface of the water. This is after almost a year of the tank operating really smoothly with zero issues. So I drop in an airstone, do a big water change, and deep-clean my filter in case there's anything clogging it up causing reduced waterflow. Once done, I take the airstone out, but the next day spot the fish at the surface again. Back goes the airstone. Overall the waterflow coming out of my outlet pipe SEEMS less powerful but I'm not positive. At this point I also reduce the amount of CO2 going into the tank. I usually try to keep the drop checker lime green, but I tone it down so it's dark green / teal.

Over the course of the next month I'm traveling a lot for work so I don't have a ton of time for troubleshooting, but every time I do a WC I try to test and see if I can take out the airstone, and nope.

So yesterday I do a WC and when I go to restart the filter, it won't prime. I can see the water levels in the intake / outlet pipes bobbing but it just can't prime. So I end up disassembling the filter a bunch of times to figure out what's wrong, and here's what I do:

• Rinse the filter media in tank water
• Remove, clean, oil, and replace all gaskets
• Remove and clean the impeller
• Take some filter media out and restart the filter in case that helps (it doesn't)
• Give up and fully REPLACE the impeller in case it's a problem that I can't see

Eventually, replacing the impeller plus tilting the filter seems to make it work. I got a LOT Of bubbles at first (thus why I decided to replace and grease the gaskets), but eventually it gets going and the waterflow even looks pretty good! I cross my fingers and remove the airstone. Less than two hours later, the tetras are at the surface again. Back goes the airstone.

At this point I have no idea what to try next. I don't even know if these two incidents are related, but I can't think of anything else that would be affecting the oxygen levels in the tank since literally NOTHING has changed.

I would welcome any ideas or suggestions! And thanks in advance. : )

 

Hello.
I think you should start doing 50percent WC because your tank is little bit overstocked...I have oase 85l tank with 12 neon tetras and two apistos and one l471+amano shrimps for keeping the aquarium algae free...I always take out 50 percent of water or even more...I also clean my filter every weak...I don't have oxygen pump, but I also have co2...do you have any surface agitation or biofilm?...put the filter output a little bit higher so the surface of the water will be moving--that is how you get oxygen...and you must turn co2 off when the lights go out...hope this helps you

 

Hello.
I think you should start doing 50percent WC because your tank is little bit overstocked...I have oase 85l tank with 12 neon tetras and two apistos and one l471+amano shrimps for keeping the aquarium algae free...I always take out 50 percent of water or even more...I also clean my filter every weak...I don't have oxygen pump, but I also have co2...do you have any surface agitation or biofilm?...put the filter output a little bit higher so the surface of the water will be moving--that is how you get oxygen...and you must turn co2 off when the lights go out...hope this helps you

Thank you! I do have the CO2 on a timer so it's out at night, but I don't have any surface agitation. The surface is really clear which is great, but I can definitely get some movement going, and up my WCs to compensate.

I guess the thing stumping me is...why now? Why would a tank be stable for a year and then suddenly have issues?

 

I guess the thing stumping me is...why now? Why would a tank be stable for a year and then suddenly have issues?

Has the temperature increased lately? That would decrease oxygen levels. Or maybe your fish have grown over the past year?

 

GlennO would be correct cooler water holds more oxygen than warmer water does.
Another factor would be plants have grown over time and so, when photosynthesizing they are creating oxygen but when they are respiring at night they are using oxygen and producing Co2. With enough plants and not enough surface agitation or current( think plants working as barriers to water flow, same way a series of trees creates a wind break). You'd get localized depletion of O2 until the CO2 created at night can leave the water and the plants can begin photosynthesizing again.

It can be a combination of the summer temp, taller plants creating circulation breaks and bigger plants creating more CO2 at night that creates the "why now" situation.
Your filter may not have the flow necessary to really circulate the water, that could also be factoring in here.

With pond this can happen from mid summer through fall, especially small ponds. As the Temps rise the dissolved oxygen in the water, well the water doesn't hold it so well and still days where the surface is smooth make it even worse. The solution in that situation is to use a pump and blow the water through the air and crash down on the surface to agitate it so gasses can leave and water can take on dissolved oxygen, Or crank up an outboard or something to churn up the water real good. I've seen both methods on the pond fish farms where I am.

I keep angels, I can run my tanks at 80-82F for breeding purposes, but this time of year I know I will have issues running that warm. Especially when I water change and the water I put in is going to bump it up temporarily by 5 degrees or so and is even hotter and probably even less O2 dissolved in it, so I run them at 76-78F. That couple degrees makes a difference on the dissolved oxygen in the water.

 

GlennO would be correct cooler water holds more oxygen than warmer water does.
Another factor would be plants have grown over time and so, when photosynthesizing they are creating oxygen but when they are respiring at night they are using oxygen and producing Co2. With enough plants and not enough surface agitation or current( think plants working as barriers to water flow, same way a series of trees creates a wind break). You'd get localized depletion of O2 until the CO2 created at night can leave the water and the plants can begin photosynthesizing again.

It can be a combination of the summer temp, taller plants creating circulation breaks and bigger plants creating more CO2 at night that creates the "why now" situation.
Your filter may not have the flow necessary to really circulate the water, that could also be factoring in here.

With pond this can happen from mid summer through fall, especially small ponds. As the Temps rise the dissolved oxygen in the water, well the water doesn't hold it so well and still days where the surface is smooth make it even worse. The solution in that situation is to use a pump and blow the water through the air and crash down on the surface to agitate it so gasses can leave and water can take on dissolved oxygen, Or crank up an outboard or something to churn up the water real good. I've seen both methods on the pond fish farms where I am.

I keep angels, I can run my tanks at 80-82F for breeding purposes, but this time of year I know I will have issues running that warm. Especially when I water change and the water I put in is going to bump it up temporarily by 5 degrees or so and is even hotter and probably even less O2 dissolved in it, so I run them at 76-78F. That couple degrees makes a difference on the dissolved oxygen in the water.

This is helpful! I have central air so the temperature doesn't vary TOO much, but I do keep it close to 80 for the rams. I'm tempted to upgrade the filter to a Biomaster 250 to see if a boost in the waterflow helps -- does that make sense?

 

This is helpful! I have central air so the temperature doesn't vary TOO much, but I do keep it close to 80 for the rams. I'm tempted to upgrade the filter to a Biomaster 250 to see if a boost in the waterflow helps -- does that make sense?

Oh wow, I had no idea Rams were similar range to angelfish. In the summer, I'm doing grow out, so I don't keep spawns and put breeding and raising on hiatus, but when I breed in the winter I raise my temps to 82-83F. In the summer when I water change due to my location I can get a 5 degree rise in tank temp off a 20% water change, worse if more water is changed. it's too inconsistent to be successful in the summer so I don't even try.
I keep my tanks in the summer around 76-77F the low side of the range, so water changes stay within their temp tolerance mostly or off by 1 degree. When the tap water is coming out at 90-95F, it's not really high in oxygen. I have central air, my tanks run at 76F pretty consistent, really hot days it might hit 78F. I use heaters to raise the temp when I breed. a water change will push me up to 81-83F for a few hours though.

Actually it might be your lily pipes come to think of it.

In fact, your 200 should have come with diffuser and spray bar, if you have them still maybe change out the lily pipes for that and see if it solves your problem. It might just be a summertime issue where you need added surface movement and added dissolved O2 of the diffuser/ spray bar. the lily pipes create water flow but not a lot of surface disruption. If you have them, I'd try that before buying another filter for $200.