We just bought a second hand tank, 4' x 2' x 2', 125 gallons, 435 litres, and we are trying to establish a biological filter. We had two small (3 inch) oscars and two swordtails in a much smaller tank. Everytime we change the water in the old tank (14 litres twice a week), we put it into the new tank. We also moved the swordtails into the new tank. How long do you think it will take to establish the tank so we can move the oscars? I have a feeling this will take forever! The swordtails just look so tiny in the new tank, I don't think they will make a difference! The guy we bought the tank off used white gravel substrate, and we took this and are using it in the bottom of the new tank. Any advice appreciated!

Moving old water into the new tank isn't really the best way of cycling it. The reason being is if the filtration on your old tank is working sufficiently, you won't be transferring any ammonia over, and ammonia is what you need to kickstart a biological filter. Most people cycle a new tank using fish. Once the ammonia has been present for a certain amount of time, your beneficial bacteria will start building up in the filtration, once there is enough of this bacteria to consume the ammonia, more bacteria will start building up that consumes nitrite, once that bacteria has built up to a sufficient amount, it will then convert it into nitrate. So once the bacteria that consumes ammonia and nitrite has built up enough to deal with the amount of fish you have got, they will consume all of the ammonia and nitrite that the fish produce. This is when the tank is cycled. So really, the best way to cycle a tank is to use ammonia. Whether this be from using fish, or a fishless cycle.

I should also add that two tiny fish in a really big tank is not going to work very quickly because not much ammonia will be produced. It will work, it will just take a very long time

This may not be the most popular answer (especially on this site) but if your two oscars are really just three inches, put them in the new tank. They ought to do fine when there is that much water to buffer their waste. I would only feed them once a day while doing it, but they'll be fine. Most Oscars are farm raised at this point and are tougher that people give them credit for. Just don't let the levels get too insane. I'd start doing water changes if the ammonia touched 5ppm. Letting it get higher could have an adverse effect on you Oscar's longevity. If you don't want to do that I understand completely. Oscars are a showpiece fish, most people prefer to pamper them. However, two year ago I started my 125 with a five inch oscar and she's still doing great.

I'll add here that I was pushing 800 gph through new biowheel filters at the time. I am intimately familiar with the filters I am using and their ability to cycle. If you're using a canister or a sump (trickle filter) this advice may just be hogwash. Other folks on here are far more versed in these systems than I am.

I would agree, as long as you do water changes to stop the Amonia getting too high, the Oscars will be okay. It may take a few weeks though, maybe six or seven. I cycled the tank a few years with two Oscars and they were okay. I wouldn't advise doing it this way normally, it's only since you've got the Oscars. The only reason I wouldn't advise doing it because Oscars are so susceptible to diseases like a hole in the head. Yes they are tough in some ways, but in my experience, they can have more problems than a lot of other fish, especially if they have been exposed to poor waterc conditions for a long period of time. I don't think the filtration makes a lot of difference. If you haven't established a biological filter, you are going to get a buildup of ammonia whatever. There's no avoiding it at the very beginning, without it the bacteria wouldn't be established. Having said that, a really good wet dry system should build up some good bacteria. However, I'm not talking about these crap wet dry canisters, I'm talking about a large sump trickle filters

Assuming your old smaller tank is cycled, all you have to do is move the fish, most of the water and the filters from your old tank into the new tank. As your filter already has all the bacteria in it your new tank will be instantly cycled. If you are not planning on using your old tank leave the filter on the new tank(you cant over filter). If you are planning on using your old tank, leave the filter on the new tank for a couple months then move it back to the old tank and that tank will be cycled too

Good call delboybully... Karma +1

Thanks everyone for the great advice. We actually want to keep the old smaller tank going. Will the existing biological filter be ruined if I move the filter on the old tank over to the new tank for a few months? Cheers!

What sort of filter is on the smaller tank? Okay, you can move the filter across to the bigger tank it depends how much bigger it is. If filter isn't up to coping with the amount of water, you won't have enough filtration. You say you want to keep the small tank running but you want to move the filter across to the bigger tank. You can do this. What I would suggest you do is when you want to get the small tank running again, do a sizeable water change on the bigger tank and put the water into the smaller tank and then just put the filter back on it, by this time, the new filters on the big tank should be established. What you want to try and avoid doing is filling the tank up with completely fresh water and then running an established filter on it. That's why I would never do more than 50% water change. In my opinion, you always want some established water in there.

So, you obviously want to get the bigger tank going for the moment. To do this quicker, move the filter from the established aquarium and start running it on the new tank. In a few weeks/months when you want to get the smaller tank running, move the filter back over to it using existing water from the bigger tank. Only do this after at least two months when you know you have established a good biological filter on the new tank/filter

The filter on the smaller tank is a 500 l/h flow filter. The new tank is 440 L. The problem is that if I have both filters running on the new tank, I wont have anything on the small tank, so I wont be able to keep the swordtails in there. This means the swordtails will have to stay in the big tank, and become oscar dinner. Sacrificial swordtails!!!



Will the biological flow filter be destroyed by running fresh water through it from the new tank? Now I'm confused!

Sorry for all this bother. You're advice has been great!

What OFL means by "fresh" water is brand new "tap" water. Tap water contains chlorine or chloramides to sterilize it. That stuff will kill off all the bacteria that make up your biological filter. Some people will tell you that you need to set up a tank and run it without fish in it for a week. That procedure would be to allow time for all the chlorine to evaporate out of the water. If your water system used chloramides then that's also more than enough time for them to break down. I've you've got all the water in the new tank already (even if it's all tap water) and all the new filters are up and running, you'd be safe to put the small filter on the new tank after just a couple days.

Remember, you're putting small fish in a big tank. You're already in the best possible position to set up an oscar tank! It's pretty idiot-proof when you are doing it this way. You're doing this absolutely perfectly. Most folks make tons of mistakes and lose a few fish before they've gotten to the point where they realize that this is the right way to care for Oscars.

albino piggy. If you want to get your new tank cycled without using the filter from your old tank, you are going to have to cycle the new tank some other way. You need to get some fish in there to produce some waste to get some bacteria in the filter. This will take some time, by which time your swordtails might have become dinner anyway in your old tank

I think you have 2 options. Move all the fish, water and filter to the new tank and hope you oscars dont eat the swordtails in the next 2 months. Then move swordtails and old filter back

Or move the swordtails to the new tank and let the tank cycle over the next 2 months and hope they dont die. Then once the tank is cycled swap the fish over. Only problem with this is that your oscars may be too big by then for the filter to cope because they are not used to so much waste. It will take them a while to 'catch up'. If the volume of oscars is about the same as swortails once the new tank has cycled you will probably be ok

Thank you everyone. I think I am just being paranoid. I'm secretly falling in love with these fish. I feel like such a proud parent!!! Anyway, I moved the oscars into the new tank and the swordtails into the old tank. Success so far, I'll just keep an eye on the water quality. Thanks again!