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Hi
I have a 160 litre tank, nil ammonia, 6.8 ph (this is very stable and never needs adjusting), nil nitrites, nitrates are 40 - no matter how much water I change, it doesn't go lower. temp is 22 degrees C. I have no idea what style of filtration mine is called, I have an internal pump which creates a strong current inthe water, drags the water out and up through a box which has the carbon out at present and a thick layer of filter wool which I am changing frequently at present. I have a plant growth encouraging tube in my light. Its been there for 3 months.

The ONLY thing I have added/ changed was food. i was unable to get my usual brand and completely out so i settle for a Wardleys community food. I hate Wardleys, but we don't have a lot of options where I live. The fish wouldn't eat it. It was in under 24 hours that the fish started to die. I haven't used this food since. ... but fish food doesn't kill fish, huh?

Remaining fish are 1 tiger barb, one golden tiger barb, 2 danios (zebra), 2 rosy tetras, 4 salt and pepper catfish and one eel tail catfish. (Catfish seem totally unaffected)

Whatever is wrong with the fish has wiped out my barbs except the two remaining which don't look healthy. the tetras look fine as do the catfish. The Danios are showing signs of stress.

The fish which died were pale in their colours, not eating (I think)and very red behing their gills. After death, there was blood flowing esternally. they also showed red under the scales. They seemed to asphyxiate. They didn't gast at the surface, but died with mouths wide open.

I ALSO have a nasty groobly slimy stuff invading my tank. Its grey brown, dirty white. Sort of slimy/ fluffy. It sticks to the gravel and the plants. Don't know if it is related, but like to know how to get rid of it!! it turned up within days of the fish starting to die off. At least, it became noticeable then...

I did two 25% water changes over a week or so, then 3 days ago I did a 50% change and trimmed the leaves from the plants, cleaned everything and vacuumed etc. I pulled out a bit of driftwood with a plant growing in it and found the slimy grey stuff had made a base in amongst the stems of the leaves and I gave it a very thorough cleaning.

Some of the Wardleys food was rotting under the driftwood.

I don't know WHAT is wrong...

I treated for flukes a few months back with praziquantal.

I use water conditioner and am treating with an algae cure in case it fights the grey stuff.

I vacuum regularly and dont have an undergravel filter.

I don't know what else to tell you...

The only scenario that I can see by what you've described is that the uneaten food rotted quickly and caused an ammonia spike which killed your fish (the rotting food under the driftwood is a good sign of that). I know you've tested for ammonia and there was none, but perhaps the intial ammonia levels after feeding were too high for the nitrifying bacteria to handle and by the time you tested, they've already multiplied and consumed it.

You also say that you gave your tank a good clean and that you change your filter media frequently. The problem I see with that is that the bacteria colonise your filter media and gravel and every time you clean the gravel or change the media, you lose some bacteria. Make sure not to touch the filter media when you clean the gravel and vice versa. I also experienced minor ammonia spikes after cleaning my tank. I've started adding Stress Zyme after my cleaning routine and it seems to help as it's never happened again.

Last thing: your nitrates. Tap water usually contain varying amounts of nitrates. Test the tap water and you might see why it never goes below 40ppm.

As for the slime you're describing, it might be brown algae. It is mostly found in new tanks or tanks with low lights. This type of algae looks a bit slimey indeed. It usually disappears after a few months if the lights are adequate. You can get a pleco (try to look for something that will not grow to be a monster), as mine seem to love this kind of algae especially.

Last,but not least, if your fish don't like the food you can find locally, just order the food they like online. If you order a large quantity, you will probably save a little as well and you will not have to worry about them going on a hunger strike again.

Hope everything is fine now!