i think you need to learn alittle more about fish keeping before you step up to rays.
the amount of water you are changing has totally destroyed your tank.
i would take your fish to the lfs and get them to look after them for you.
get more filtration on your tank, even though you said you have 2 rated for 55g tanks. they are obiously not doing the job.
then i would leave the tank alone for a couple of weeks and allow the tank to cycle properly.
when you do a waterchange do you use tapwater conditioner? do you measure it out correctly for the amount of water your put back into the tank?
once the tank has finished cycleing you need to stick to a decent timetable for waterchanges and tank work.
to much can be just as bad as not enough.
40% WC once a week, useing the correct amount of tap water conditioner.
filter should be left alone untill you notice a decrease in flow rate, then do one filter each week to allow the cleaned one to catch up ready for the next one to be cleaned. when cleaning filters, just take out the sponge and clean it gently in a bucket of tank water (not tap water!). they dop not need to be spotlessly clean just a little squeze out to get rid of the thick thats it. if you run filter floss change that for new stuff.
NEVER do a full tank strip down and clean as you are just undoing all the hard work of the cycle.
small amounts of cleaning often to keep things ticking over nice.
please dont get a ray untill you have a good amount of expereance as they will not put up with any problems and it will just suffer.
get the bigger tank up and running and well cycled first.