Oxygen content or chemical problem?

Ok, I regularly do water changes 1nce a week, 20-25% each time on my 56 gallon

Awhile back, I made the mistake of using seachem's PH Neutral. I thought it was good, cause it lowered, I hoped anyway, my hardwater PH
well, sure it did, but along with that came massive algal growth on my plants.
I read the container, sure enough, contains phosphate.
but, I was extemely busy and even fitting in the water changes was tough, but I stuck to it
soon, the water started to get that icky film on the surface. Mine was so bad, that I could watch it fight the surface agitation I used to try and break it up
well, my busy period ended and i finally got a chance to clean the tank properly, made myself a surface extractor, and got the film cleaned up
howver, I noticed that the fish, at certain times, all of them would be at the surface gasping for air. I tested the water, all good, no ammonia/nitrite
after I cleaned the tank and filter, and hooked up a new filter (fluval, weee) and changed to Amquel, the surface film was completely gone, and everything seemed good
but, it seems now, that when I change the water, and if I fill the tank past the output of the filter(s), within a few hours they are all gasping at the surface again.
I test the water, but I cant trust the reults cause using my test kit for ammonia the water goes grey and silty, hehe, no help there
but, if I move the spraybar and fluval output to disrupt the surface, in a little bit, they arent at the surface struggling to get oxygen, but they arent having a good time either

anyways, my tank is planted, so that disuption isnt my friend, at the same time my fish have to live!

HELP! Is my oxygen content permantly sabatoged since the surface film, or what

more info
PH in tank 6.6
ammonia 0
nitrite 0
nitrate unknown
and these episodes ONLY happen after water changes

maybe its a PH spike...come to think of it...cause I have peat in the fluval to help soften the water, works awesome, fish are much happier, now that I think of it, the PH in my tap water is like 7.6, could that be causing the stress to the fish when I change the water?? or is it just a gas exchange problem?:sad :sad :sad :sad

*bump*
I know its quite a read, but any help please:shake:

the one with phosphate, is the one I WAS using

pants are doing better, since I started adding ferts.
Now, I thought that the ferts and dechlor might be interacting in some way, but, I was addding the ferts long after this original problem was happening

no, no oxygen testing for me
but, it seems also like my tank is unable to KEEP oxygen in it as well, ida know, still hoping for help, not to say you havent helped so far, heheh

umm, that question may be a bit of a sidetrack, I dont think the lighting has something to do with the fact this only occurs at water changes. I would think it might be the case,cept I dose the tank 2 times a week and, like I said, it only happens when I change the water, not during the dosage mid-week, but, ya never know

*bump*