The white fish with orange head looks like a Koi carp but it doesn't have any whiskers. They usually have a pr of short whiskers under their mouth. It might have them pushed against the skin.
The koi doesn't look that big, I'm guessing it's about 2-3 inches long.
There appears to be a white male betta with a damaged tail, and a male orange and red guppy.
These fish with the 5 tetras shouldn't be a major problem in that tank. However, the koi will need to go soon because it is going to grow too big for the aquarium.
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The easiest way to reduce ammonia levels in a tank is to do a huge (80-90%) water change and gravel clean the substrate. The following day, start doing a 75% water change each day until the level is back to 0. Big water changes won't upset the filter bacteria or fish as long as the new water is free of chlorine/ chloramine before it's added to the tank, and as long as it has a similar chemistry (pH & GH), and temperature to the tank water.
*NB* Make sure you turn the heater and filter off before you do a big water change, and turn them back on after the water change.
*NB* Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
You should also reduce feeding to once every couple of days until the ammonia level is back to 0.
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5 tetras, 2 Bettas (male & female), 1 male guppy and 1 small koi carp should not cause a massive ammonia spike in that tank. There are compatibility issues with the fish but I will deal with the ammonia first.
If the tank has been running with a filter for one month, it should have developed the first group of beneficial bacteria that convert ammonia into nitrite. If you replace the filter media (materials) on a regular basis, you will remove these good bacteria and have ammonia problems.
What sort of filter do you have and how do you clean it?
Filters should be cleaned at least once a month and every 2 weeks is better. However, do not clean a filter for the first 6 weeks unless it is really dirty and the flow rate is reduced, because you can wash out the new bacteria and interfere with the cycling process.
Power filters usually have sponges in them and these should be washed out in a bucket of aquarium water. If your filter has a pad in it and you replace the pad as recommended on the packaging, stop doing that. Most filter pads can be squeezed out in a bucket of tank water and re-used a few times. You can also replace the pads with sponges. There are lots of different brands of power filter and you can get replacement sponges for them. I like AquaClear sponges but there are plenty of other brands. Use a pr of scissors to cut the sponge to fit in the filter. Do not change all the pads at once. Put the new sponge in the filter with the pad and let them run together for at least one month, then remove the pad.
Sponges will last for at least 10 years.
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Compatibility issues.
Depending on the species of tetra, they might be fin nippers and chew up the male Betta. A photo of the tetras will help us identify them and consider them safe or fin nippers.
Male & female Bettas don't always get along and will fight if put together and they don't like each other. You need to watch them closely to see how they get along and if one is hiding, then separate them.
Guppies come from water with a GH around 200ppm and a pH above 7.0.
Bettas and tetras come from soft water with a GH below 150ppm and a pH below 7.0.
Koi carp usually do best in hard water that is similar to the guppy's requirements.
What is the GH (general hardness) and pH of your water supply. This information can usually be obtained from your water supply company's website or by telephoning them. If they can't help you, take a glass full of tap water to the local pet shop and get them to test it for you. Write the results down (in numbers) when they do the tests. And ask them what the results are in (eg: ppm, dGH, or something else).
If you have soft water, you should get rid of the guppy and koi.
If you have hard water, get rid of the tetras and Bettas.
However, it also depends on the actual GH and pH. If the GH is around 150-200ppm and the pH is about 7.0, they should be ok together from a water chemistry point of view.