I also think you're at a point where you don't need to change water as often as you are... this will only affect your ph. Try to move to once a week. Your levels are at zero so once a week with your load is more than enough, especially with two filters each of which is rated for your tank size.

I'm wondering why your nitrates are back at zero... maybe you pushed into a mini cycle... did u change a major bacteria colony? this could lead to your fish dying (well the weaker ones atleast).

As for feeding, its rare to underfeed and i think if predation was the cause you would have seen foul play earlier than today. Not to mention you said they ate last night, so that rules starvation out.

I agree with Miami that ph could be it, so for now, less water changes and don't introduce new fish until you find out what caused the loss. Good luck

ok not sure where to start, stop using chemicals, the less chems you use the better, all of my tanks have cycled naturally i don't believe in using chems in order to cycle your tank anything worth doing is worth doing right the first time, so your issues with the ph spikes are due to one, too frequent of water changes, two not enough charcoal in your filtration, three the chemicals you are adding to the tank, slow down, take a step back and be patient, if you are finding your tap water is too high in the ph scale buy filtered water, secondly clean your filter very well, also because you have added live plants make sure you don't have rotting plant material at the bottom of your tank, if the roots are brown and discolored and look as if the ends are blunt this means the roots are rotting and the plant is in poor health. at this point if i were in the situation you are in, i would do a 50% water change and let the tank sit like that for a week or two, make sure the filter is cleaned and there is fresh charcoal in there, get an ammo carb pad or get a small piece of driftwood/bog wood to put in the tank, this will bring down the ph over time, do not add any more fish, also if you are using a bio wheel filter throw it out, i find they are garbage and don't move enough water through the filter media to really do anything important. aqua clear makes a hang on filter that uses a sponge, a carb bag, and usually has room to add extra filtration, get an ammo carb bag and put that in there, this will move around enough water and get your tank cycling in the right direction it should start reducing the ph, i can't stress enough that when you add chems to your water you may temp cover the problem however it doesn't fix it as your filter filtrates the chems out as well as evaporation, the gas exchange from your tank will eventually expell the chemicals from the water. so there's no point. i hope that this information helps, i don't mean to be so short and forward but this is how i have handled situations similar in the past, i have helped numerous people set up tanks and always use a slow natural cycle, with seeded water from my 125 gal tank, about five gal per 30 gal, i add my water to the tank and within three weeks the tank has cycled and is ready for fish.
Brandon

Thank you all, I'll do these things right away. I just added more charcoal and change it out every 2 weeks. I keep two Penguin 150's on this 28 gallon (with one pad each) with an extra filter pad in one, and a charcoal bag in the other. I got that from another thread. Even if my fish are dying, you sure can't find any clearer water...valuable lesson.

Zambize

I'm no expert but i don't know if throwing the bio wheel out at this point would do more good or do more harm... i haven't heard of too many people having ill effects from a bio wheel, plus there is the filter cartridge which receives plenty of water through it, so the wheel is only added biological filteration.

I also feel a huge water change at this point will only drop your ph once again... im not hardly as experienced as brandon (poetic irony), so i'm not knocking down his advice, i just dont know if that would help. Maybe he's seeing an angle that i'm not?

Great, I'm going out of town and will only be home the next 2 nights, then be gone for 3 days. I'll do the recommended water changes with r/o for those days though. It sounds like that will help for now. Hopefully the weaker fish have died and the ones remaining can tough it out. I've heard that all hell breaks loose only when you're going on a trip. And on another thread, I've got a mysterious *thing* growing on the outside of my tank. Sheesh. It's never boring, I'll give ya' that!