OK, gotcha! First, we need a reading for nitrites...you gave it for ammonia and nitrates but to know if the tank is still cycled we need the nitrite reading.
ICK - Turn up your heater.....slowly to 83.....just raise the temp about a degree every hour. If your heater can't handle that, you may need to add another heater for awhile. HOLD the temp at 83 for at least two weeks. This will kill all the ick. While you are doing this you will need to be doing 25-50 % water changes every other day MINIMUM...I put it that way, because of your second problem....HIGH nitrates.
40 is twice the safe limit on nitrates. You know this and I can see you are concerned, but you have to up the number of water changes and the amount to get them down, and keep them down in this tank. After the ick is gone you should continue with 50% changes twice a week or how ever many it takes to get the nitrate under 20.
Algae is a normal state of affairs with high nitrates. The two things you can do is to cut the nitrates in the tank and limit light exposure. If you have lighting, only have it on 8-10 hours a day. If your tank is near a window, move it, or pull the shades during the day. The diatoms are due to the severe filtration and gravel changes.
The rule of thumb for HOB filtration is 10 X the gallons of tank turn over....so if you have a 55 gallon tank your filter has to process 550 gallons per hour. The Aquaclear processes 200 gph. Two of them will only do 400 gph. Your new filters are not processing the water fast enough.
A JD can be in a 55 gallon tank MINIMUM, but with the load you currently have
you need a much larger tank. Without the pleco your adult fish need a tank bigger than 55 gallons. If your pleco is a common, which is usually the only kind that would be sold for an aggressive tank, it has to be counted for up to 18 ". The babies another 100 inches.
At this point you need to do several things besides the water changes. You need to get a bigger tank, with adequate filtration. To start you could get a Aquaclear 110 use it in your current tank WITH the filters you have running, then look for a tank that is 100 gallons, and use the 110 in that once you get it set up.....instant cycle! You would also want another filter in a bigger tank, like a Eheim 2028 or bigger. Then the filters you are currently using could stay with the 55, with a less heavily stocked tank.
The nature of the fish you have causes more tank mess than the normal community tank, they are messy eaters. This adds to the problems of nitrate buildup with less than optimal filtration.
I wouldn't "change" filter media or gravel, the bacteria needed for the cycle are in the filter and gravel. If you do anything with that gravel it should be to vacuum it every week. With each water change you can vacuum 1/2 of the tank. This gives the fish places to go away from where you are working. As long as the tank has been set up, I'm sure it could use special attention to this chore.....Good Luck, and keep us posted.
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