08-04-2009, 12:53 PM
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Problems are sometimes difficult to diagnose, but I'll offer a few suggestions.
I'm thinking that the stocking level may be high for this tank. It's a 240 litre (approx 50 gallon) and while you don't say how many of each fish species you had, the number of deaths indicate there may have been more than safe. When problems occur, a higher fish density can often aggravate them, and the fish may feel too cramped and this causes stress, particularly if there are any slightly aggressive fish in the mix. And water quality is more fdifficult to maintain if the bioload is beyond the normal capacity of the filter and biological equilibrium.
Angels and neons are not a good pairing, as the neons can tnd to harass the angels (fin nip those flowing fins) and angels do eat neons if the neons are small enough to fit in the angels' mouths. Sharks can be aggressive particularly with each other and with other bottom dwellers, so this is something to watch out for and perhaps avoid. If the "clown" is a clown loach, this fish needs to be kept in a group as they are very social and shoaling by nature; 5 would be good, but not in a 50g tank. These fish will grow (if they are kept healthy and given the space) to 12+ inches. Kept in too small a tank, they (and all larger fish) may be stunted, where the internal organs try to grow even though the outward body can't due to the lack of space and associated water quality; this again brings on stress, and frequently causes immune system issues. I would find a new home for the clown loach (they store may take it back for credit, or another aquarist?).
The pH reading is OK for livebearers which are fish that prefer basic (slightly alkaline) water. I'm confused by two readings--is "A" and "B" different test kits? The nitrate reading "0" is odd, with this fish load and only bi-weekly water changes the nitrates should be showing something; it is normal to have 5-20 ppm of nitrate; some fish are fine with slightly higher nitrate levels, but it is generally deemed safer to keep nitrates below 20 ppm, and the weekly partial water change does this.
Partial water changes are best when carried out more frequently even if less water is changed. Weekly changes of 30-40% would in my view be ideal, but even doing 25% every week would be better than every second week regardless of how much water is changed. The aim is to maintain a better stability in water parameters, and replacing water more often removes toxins like urine which no filter can remove; only the water change can handle this task, and the more the better for the fish's health.
As for the immediate problem, I would do more frequent partial water changes, maybe twice a week, for a couple of weeks. And consider any changes to the current fish as mentioned above.