I've been experiencing nasty problems after every water changes since setting up my Oscar tank 6 months ago, and decided to set up a dustbin and filter conditioned water for a week or so before using it for the partial water changes in the big tank. The bin has an internal filter and heater, and is treated with PRIME at the normal recommended dose.

I haven't performed a water change in the Oscar tank for the past 14 days. Too long I know, but I've had flu and been feeling crud. Anyway, this morning I propose to do the necessary water change but having tested both water in the Oscar tank and the bin am concerned about commencing.

Oscar tank:

Temp: 25 C
pH: 6
Ammonia: 0
Nitrite: 0
Nitrate: 5 ppm
GH: High (Added 19 drops of API GH test to get colour change)
KH: VERY low (Added 1 drop of API KH test to get colour change)

Bin:

Temp: 25 C
pH: 8
Ammonia: 0
Nitrite: 0
Nitate: 0
GH: High (Added 13 drops of API GH test to get colour change)
KH: 7dKH (Added 7 drops of API KH test to get colour change)

Tap Water:

pH: 7.2
Ammonia: 0
Nitrite: 0
Nitrate: 0
GH: High (Added 13 drops of API GH test to get colour change)
KH: 9dKH (Added 9 drops of API KH test to get colour change)

I am concerned by the pH differences and the over all high GH, and the low KH in the Oscar tank.

I've API pH UP, API pH DOWN, API Proper pH 7.0 and Aqua Hydrotech Buffer Up! in the cupboard here. Not sure whether the treat with any of these or what to do about the GH annd KH levels.

Any suggestions from the more experienced in this area about what I should do?
Many thanks.

Okay, the reason why there is a difference in pH between your aquarium and the bin containing water is because you don't have any fish in it. Once you have fish living in water, they will have an effect on your pH.

Not concerning your KH on GH, I'm not sure if there's much you are going to be able to do to bring this down to something like 50, I believe this is what they recommend. American fish, but please don't quote me on that. the water readings in your Oscar tank are absolutely perfect. How are your fish? They will tell you if they having problems with the water, you can be sure about that.

Just briefly returning to your bin containing water. If there are not any fish than expected readings to be completely different to a tank containing fish.

"Just briefly returning to your bin containing water. If there are not any fish than expected readings to be completely different to a tank containing fish."

Okay, but could the difference between the parameters in the Oscar tank and the water I add during water changes (i.e. much higher pH and higher KH water in the new water) be causing the fish deaths and gasping previously experienced after every other water change I've ever done?

I'm desperate to avoid the inevitable problem of the Oscars gasping, and then floating sideways at the top of the tank almost dead after I use fresh tap water (conditioned before adding, of course). It's horrible. Thankfully they pick-up after 3 or 4 days if left (hence why I hoped filtering fresh tap water in the bin few days before adding it to the fish tank, might help.) But looking at the test results, the water in the bin has increased in pH from the fresh tap water reading. Looks worse to use to me. Deep sigh.

Is it safe to use either the bin water or Prime treated fresh tap water, going by the above readings? As I say, after every water change the fish really suffer and I just don't know what to do. I'm starting to DREAD doing it. If so, what percentage change and how often?

The Ozzies are fine ATM, as are their two Angel Fish companions. The water has yellowish tinge, and I've fine brown algae growing around the edges of the glass. I need to do a water change but am anxious.

how large is the tank, and how much of a water change are you going to do?

340 litres and I was planning a 20% change. I normally perform one of these once a week, but like I've said - the fish really suffer.

I'm wondering if smaller but more regular changes would help?

If your fish appear to be healthy at the moment then I don't think you should worry too much. What test kits are you using? I have to say that your tank is not big enough for two Oscars, it is approximately 75 gallons which is big enough for one Oscar and maybe a couple of smaller tank mates.

I've got to be perfectly honest I don't think you will really benefit from storing your water in a bin for a week before adding it to your tank. As long as you used a good quality water conditioner you can add the water straight to the aquarium from the tap, I've been doing it for years and haven't run into any problems.

I would certainly do smaller and more often water changes. Make certain you maintain your filter well, and vacuum the gravel. this could be the reason your tank PH/KH is very low in relation to the tap/bin water.

the PH difference from tap to bin is normal. you may want to consider a buffer for the KH. or put a aragonite bag in your filter.

what type of gravel do you have? do you have a lot of bog wood?

Yes, you're quite right about the tank not being big enough. We're actively looking for a decent sized 5 foot tank (second hand) but they're hard to come by. I'm keeping an active look out, and the ozzies will be up graded ASAP. Meanwhile I am running two big external Eheims, a Fluval 4 Plus internal, a surface agitator, 'vacuuming' the gravel weekly and performing the 20% water change every 7 to 10 days. The fish are keeping well, except following water changes.

I know what you're saying about simply using the water from the tap. I've also been doing this for years in my present home: with the mixed tropical tank (a 38" one) and the Malawi tank. I've never experienced any problem. However, the same tap water - treated with the same water conditioner (Prime) as in the other two tanks, results in MAJOR problems in the Ozzie tank. I just cannot fathom it.

Immediately after the water change the fish in the Oscar tank appear fine. Next morning (about 18hrs later), however, the more sensitive fish will be dead (I've lost Pictus and Clown loach) and the Oscars will be listless, floating sideways, red-gilled, rapidly breathing and gasping at the top of the tank. This happens after EVERY water change, no matter how big or small. NB: The other tanks in the house, using the same water at the same time, are never adversely affected. It's a mystery why the big tank keeps having this problem, whilst the two smaller ones in the house dont.

Again, I know what you're saying about the conditioned tap water - but for some reason that just doesn't work for me ATM.

"what type of gravel do you have? do you have a lot of bog wood?"

I'm just using normal pea gravel in the Ozzie tank. I added a large peice of old bog wood last night infact. Prior to that, just graphite rock as decoration. I know bog wood can lower pH but THAT quickly!!! Over night? Unfortunately I didn't test the water before adding the wood yesterday, so don't know whether the pH drop is directly attributed.

The pH in my mixed community tank (using the same tap water, Prime and filters) runs at 7.2 and pretty much stays there. A pH of 6 in any tank in my house is REALLY low for us here, so I was surprised the Oscar tank read that low this morning. The bog wood I put in the Oscar tank last night was actually taken from the mixed community tank where it's always lived. It'll be interesting to see if this affects the pH of that tank now it's been removed.

I was thinking it could be your gravel? I remembered your post from last time about the PH drop and the problem after 18hrs. there are a few possibilities. if i was you I would remove each one at a time slowly.

1 gravel. I would consider removing to bare bottom to see if thats any better.
2 remove any plants or wood.(I know you just put it in) but you don't want anything to drop the PH further.
3 your filters. make certain you have full media, enough to cope with the ammonia spike from the water change.

What are the Ethiems you have?whats in them?how much media?

ok the reason I suggest these are, removing the gravel will make the tank cleaner. No crap hiding under it, also it removes the possibility that its made of anything that drops the PH.

any crap held within the gravel or filters will drop the PH if its left long enough. make sure you clean your filters regular. ammonia spikes are common if your filters are not large enough or have sufficient media to deal with the extra spike. you could try using extra "Prime" 10-12 hrs after the water change. a double does would not hurt the fish and would take care of any ammonia/cholramine that hasn't been broken down by the filter. you could also try adding some crushed coral or aragonite.

another thought just came to me, your niTRATES are very low (5pph) for 2 large oscars in a 75g tank with 20% water change every 7-10 days. double check you filter size/media. its very lightly its not enough filtration or media inside. if you have large filters packed with media maybe they are not cycled fully. cos your nitrates should be much higher given tank and fish stock? check both filters are working fully at full flow. somethings not right because the nitrates are not building up at the right rates for tank/fish stock given your maintenance.

Just a quick update.

I removed all but 1cm of the gravel from the bottom of the Oscar tank this weekend; removed the bog wood and rocks, installed a powerful bubble wall and serviced my Eheim filters. I also added carbon to one of the filters, and a Purigen pillow into the other.

Then I did a 20% water change, added conditioned water straight from the tap (set to 25C) and using Prime and...... dare I say it, all seems well.

The Oscars are just LOVING the bubbles and everyone's looking perky and well.

So, many thanks for the advice! Seems I've cracked the water change problem with your help.