Tank water is cloudy

Hi people.

I got extra nervous now, and even my fiance is having a lough at me that I should be going crazy by now, and in fact I am.
Dont know if you read my last topic, but I have 3 old angels in tank and they been there for a while. So I decided to get more of them and got 5 little ones. I knew they may die so hoping I get more will leave me with a chance of having 2 of them to survive. My bad, didnt realised that old ones wont like this company and they killed all of them within 36 hours. it was my very first and really painful experience. So back to cloudy water. I see 2 possible factors that may have impact on cloudiness:
1st - Before I got my little ones I have changed gravel in to sand. I got them of viovet shop online, special sand, washed it even it says its clean. water got clear within 6-7 hours and looked perfect to me.
2nd possible factor was that when I got littles and noticed them acting funny I started to think if they cant eat normal food ( TetraPro algea and frozen blood worms) I added NovoTom atea food for fries.

gap between this to things is about 4days. now its been nearly 2 weeks, I done 2 of 30% changes and one as nearly as 90% change and water was getting clear for a while but next morning it was back to same cloudnes.
In a tank I got 3 Corycatfish, Pictus catfish and Pleco. they seem to be raising sand a bit. I have got air stone too, but since I saw it digging itself under sand I lifted it on to peace of wood so its not raising sand any more. I got fluval internal filter, which is about 5inch above sand, with new sponges and bio things inside for a week now. and even when I type catfish is all laying and chilling without messing up the sand. HELP ME WHERE TO LOOK OR TELL ME WHAT I DO WRONG THO.
p.s. i got 5 in 1 test kit and only problem in tank is Gh and Kh Ph - 7.5 NO2 and No3 - 0.

Thanks guys
Will

I'm no expert, but it sounds like it might possibly be a bacterial bloom, again clutching at straws, maybe caused by you changing your substrate? Good aeration might help here.

Young Angelfish can be very sensitive to a change in environment, I wouldn't necessarily blame the older Angels unless of course you witnessed it? I myself added 4 youngsters to two older and much bigger ones, its now the youngsters doing the bullying