Can someone help me with flow advice?

[mangeltrueman]

Hi all,

Been struggling a bit with some BGA and was thinking it could be something to do with the flow in my tank; if someone could comment on what they think, that would be very helpful. I've done the usual nitrate increase, 3 day blackout, increased oxygenation, treating with chemicals.... but it still keeps on creeping back in patches throughout the tank and its starting to frustrate me a little. I thought I'd nailed root cause with low nitrates and bad oxygenation, but to no avail.

Tank is a 120 litre Fluval flex. The output from the rear filter area is through two duckbills that you can see in the picture below. The flow into these duckbills is provided by 2x Eheim compactON 600s These two pumps run in parallel and are joined with a y-connector. One side of the y-connector goes through a co2art inline co2 diffuser before joining the other side. The duckbills are pointed out and down to try to get co2 towards the bottom of the tank and into the corners. I've also got a 300lph skimmer on the left of the tank that faces forwards on the left hand side. This also provide aeration overnight.

What I'm thinking of doing is replacing the duckbills with a long spray-bar across the width of the tank as per advice that I have seen mentioned elsewhere on this site from the likes of Clive. I understand the idea that a good rotational flow throughout the tank is good, but wasn't sure how well that would work with the larger rocky section on the left hand side of the tank which obviously blocks flow from the front to the back. I've got Rotalas (green and orange juice) behind this which currently are doing ok-ish, but the orange juice seem more affected by the BGA than the normal green Rotala. They do get a bit of flow from the downward pointing duckbills at the moment, but i fear i might lose that with a spraybar. My monte-carlo is also affected in places. Also not sure if the skimmer will provide interference to the flow in a spray bar setup. The outlet for that is about 1/3 of the way down the tank.

Any thoughts or advice would be gratefully received.

 

[Gold Fish]

You are correct in changing to spray bar, but have two, one for each pump. By having them in a y connector is not good. They are fighting for the same cross section of a pipe. Have them feed into each spray bar, one going left and one right. You just need too make another hole next to the existing one. Put the pumps facing each other and between them add the co2 diffuser such way it will be suck both sides. And also you are not saying anything about what you have for filter media. If it's the standard arrangement, then it is not sufficient. You need to be able to remove the ammonia as quickly as is produced.

 

[mangeltrueman]

You are correct in changing to spray bar, but have two, one for each pump. By having them in a y connector is not good. They are fighting for the same cross section of a pipe. Have them feed into each spray bar, one going left and one right. You just need too make another hole next to the existing one. Put the pumps facing each other and between them add the co2 diffuser such way it will be suck both sides. And also you are not saying anything about what you have for filter media. If it's the standard arrangement, then it is not sufficient. You need to be able to remove the ammonia as quickly as is produced.

Thanks for the response.

The co2 diffuser is an inline diffuser so it can't go between the two pumps. Not sure what difference filter media makes to this arrangement? The pumps can always put out their full LPH as they are inside a baffled area with the two media areas to the side, like the image below (which has one pump whereas i have two). I've got all 4 areas filled up with biological media. There is no input restriction on water going into the pumps at all.

As for the Y join and the use of two spraybars, it does bare thinking about. I'm just using what came with the tank other than the two pumps instead of one (as i had them lying around). I don't think a y connector reduces the flow too much as its the same diameter as the output of the pumps themselves, but yes, it will reduce it for sure. The whole setup is designed around 16/12 hose, so the ID of the duckbill outlet are just as restrictive as the y connector. Be interested to know what the max flow rate is through a 16/12 hose connector...

I guess in terms of flow rate, the better option would be to replace the whole lot with one more powerful single pump with a larger ID pipe. Or possible split it into two spraybars, each with a lower flow rate as you suggested but then i need to consider my options on another inline co2 diffuser as i dont want CO2 just coming out of one spraybar..

 

[Gold Fish]

Don't get upset on me, I'm just trying to help.
Do the following experiment and decide by yourself: lower the water level to expose the duck bils and make them arching the water expelled. Mark the distance. Now stop one of the pumps and compare the lost pressure and flow. Then imagine that you can have twice as much as that smaller flow, if each pump will have it's own tubing.
Regarding the media. The water take always the easiest route, through your sponge around your media. Move your media into vegetable bags (see Asda or other shops) and put them at the base of the sponge compartments forcing the water through them. Let the sponges on top for additional filtration especially that I suspect there's the most bacteria at the moment.

 

[mangeltrueman]

Don't get upset on me, I'm just trying to help.
Do the following experiment and decide by yourself: lower the water level to expose the duck bils and make them arching the water expelled. Mark the distance. Now stop one of the pumps and compare the lost pressure and flow. Then imagine that you can have twice as much as that smaller flow, if each pump will have it's own tubing.
Regarding the media. The water take always the easiest route, through your sponge around your media. Move your media into vegetable bags (see Asda or other shops) and put them at the base of the sponge compartments forcing the water through them. Let the sponges on top for additional filtration especially that I suspect there's the most bacteria at the moment.

Upset? Sorry if I came across as upset, didn't mean to.

I'll do something like that to test the flow rate. maybe see how long it takes to fill a couple of liters in a jug with one pump and two and compare the results.

Never really thought about moving the media around but will give it a go and see if it makes any difference to ammonia take-up

Thanks

 

[mangeltrueman]

I swapped out the duckbills for a spray bar today. Flow seems much more uniform, especially to the front of the tank and down to the carpet. I was thinking of maybe adding some holes pointing backwards as well to get a bit of flow toward the back of the tank as I am conscious that the circular flow is likely to be blocked by my hardscape. I've got some spare pipe so might try it. If anyone else is thinking of doing this on the Fluval Flex tanks, the threaded connector through the rear filter compartment is a 1/2 inch BSP, so you can get connect into that with 1/2 inch solvent weld pipe (which i havent welded, interference fit seems to hold well enough)