Tank in use - no filter

[siwq]

So for the time being I'm caring for a filter-less aquarium of some folks I know.
It's a 15G tank that was cycled with just an oxygen pump for a week and has an airstone in it and two three pipes that throw out oxygen.
There are 2 small koi (I don't even know if these really are koi actually... they're small and not so thick like those Japanese koi look like. One has tiny stache like the koi do though), a comet and a platy (over stocked and incompatible fish, I know been telling them... but apparently the fish shopkeepers didn't tell them about the incompatibility and they were keeping the platies without temp. regulators too and in not so good conditions so they think that returning the platy would be worse.. I think they know platy are group fish but its pair died and they didn't want to get more since I'd told them by then that their tank was over stocked. I don't know what to tell them to do now. I also don't want them to return the fish because I know what sort of living conditions these shopkeepers near us keep the fish in these folks are prolly keeping them in much better conditions in comparison because they do water changes often, while the water's green in some shops' aquariums... :\ Please do give an opinion on this..)
I was thinking maybe a filter would help make the aquarium a better place for the fish. What sort would be best for a 15G tank? I haven't brought up the idea with them but I just wanted to get some knowledge before I talked to them about this. Does the filter have to be up and running for a while before the fish are put in? Or can we just use it with the fish in right after it has been installed?

 

[Colin_T]

Hi and welcome to the forum

Can you post a picture of the tank?
If it has a couple of tubes with airbubbles coming out it might have an undergravel filter. You can move some of the gravel near the tubes and see if there is a plastic plate with holes or slits in it underneath the gravel. If there is then the tank has an undergravel filter in it.

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If the tank does not have a filter then keep feeding down to 2 times a week and do a 75% water change and gravel clean the substrate 4-8 hours after feeding.

If the tank does not have a filter in it, then you can add one while the fish are in the tank and the filter will develop beneficial bacteria over the next month. Keep feeding down to 2 times a week and do water changes as above until the filter has established in about 4-5 weeks.

 

[siwq]

Hi and welcome to the forum

Can you post a picture of the tank?
If it has a couple of tubes with airbubbles coming out it might have an undergravel filter. You can move some of the gravel near the tubes and see if there is a plastic plate with holes or slits in it underneath the gravel. If there is then the tank has an undergravel filter in it.

--------------------------
If the tank does not have a filter then keep feeding down to 2 times a week and do a 75% water change and gravel clean the substrate 4-8 hours after feeding.

If the tank does not have a filter in it, then you can add one while the fish are in the tank and the filter will develop beneficial bacteria over the next month. Keep feeding down to 2 times a week and do water changes as above until the filter has established in about 4-5 weeks.

Thanks! Sorry I forgot to mention yes it has an undergravel filter. There are three four plastic plates. I removed one while doing a massive water change a couple of days ago and cleaned a bit of the filth beneath.
So if a filter is installed, with the undergravel filter, should I lessen feeding time? They were fed twice a day before but I only feed once because I thought they were being overfed and weren't excreting properly.

 

[Byron]

An additional filter will do nothing beneficial here. One of the pros of undergravel filtration is how good it is. Having said that, incompatible fish are still incompatible fish and filters will not change this at all.

Goldfish cannot be combined with tropical fish. A 15g tank will not last long for goldfish so this is going to be an issue very soon. And again, another filter has no bearing on this, as it will not increase the tank physical space nor the water volume.

 

[Colin_T]

If the filter has been running for more than a month, then you can feed 2 times a day if you like but the fish will be fine with one feed a day.

If there is a gravel cleaner (like the one in the following link) you can use that to clean the gunk out of the gravel and from underneath the filter plates, when you do a water change. Just start water syphoning out of the tank and then push the gravel cleaner tube into the gravel and hold it there for a moment. As the water drains out it will suck the gunk out of the gravel and from underneath the filter plates. Clean as much gravel as you can while draining out 50-75% of the water. Then fill the tank up with dechlorinated water.
https://www.about-goldfish.com/aquarium-cleaning.html

As Byron mentioned, adding another filter won't make any difference if there is an undergravel filter in the tank.