75 gallon uk tank im running a eheim professional 2226 external filter and a fluval internal filter,my ammonia level keeps spiking. today i done a 50% water change on it and tested my ammonia level and its realy high. i have put ammo lock in it.my nitrate and nitrite levels both read 0.any ideas what is doing this?

oh i forgot to say all i have in it is my 2 oscars and a small sucker fish

Okay, it could well be that your filtration is not quite up to keeping up with the bio load that your fish are creating, how big are the Oscars?. Your Eheim filter is I think rated for a 77 gallon tank. The internal filter is not going to be rated for much more than a 30 gallon tank.

Have you turned your filters off for any period of time lately, or have you cleaned the media? Remember that ammo lock does not actually remove ammonia, it just renders it non-toxic, so when you test, you will still get a reading of ammonia.

I would suggest that you stop feeding altogether for the next day or two, try and reduce the amount of waste going into the water. Don't keep carrying out huge water changes several times a day if that is what you have been doing, one water change a day is sufficient. Hopefully the ammo lock will do what it is supposed to do so all you need to do is wait for the bacteria to catch up and start removing the ammonia. The fact that you have still got a zero reading of nitrite indicates that you haven't actually got a problem with your biological filter, if you did, you may be also getting a nitrite reading.

how long have you had the tank set up and running with fish?

the oscars are 12" and 9" i clean the internal filter 2 days ago and turned all filters off for half hour whilst doin water change today.i havent cleaned external filter for about 3wks

it was a second hand tank and filter but its been up and running for well over 3months

ok, it looks like your cleaning was just a little more than needed. did you use tank water to clean? like ofl said the ammonia lock should make the ammonia harmless and the filter will take care of it. if you do a water change make sure you replace the ammonia to cover the new water.

ok thanks for help. i normaly clean 30% once a week.also would a sump filter be better for the tank?

i am very surprised your nitrates read 0. you may want to recheck this??? has got me wondering a bit. how much water have you changed in recent days, or should I say since you noticed the ammonia spikes?

il recheck nitrates and let u no.the ammonia spikes have been happening for about last 2 wks.i only been doin water change as i normaly would at 30% once a week

nitrates are upto 5ppm.

so the problem started just after you cleaned the internal filter. as ofl said its the smaller of the 2. I would recommend getting another larger filter. I think you have just got enough filtration when all is working well but when you clean them, its leaving them unable to cope.