Need Help with new, not-eating Discus, please

[discus_gal]

Hello,

We are new to this forum (and to Discus for that matter) and need help with one of our new fish.

Last Monday we got six new Discus. Four of them are about 6 months old, two of them about 10 months (4" and 5").

We have a 65 gallon (35x18x24) Discus-only tank, which was cycled for a few months with serpae tetras and guppies before the Discus family moved in.

Here is what we have:
Fluval 406 Filter (no carbon)
2 150W Cobalt Heaters
TopFin Medium Fine Bubble Air Stone
UV Sterilizer Lifegard Aquatics 24 Watts, which was just installed 7 days ago (2 days before we got the Discus) and does not seem to be doing its job
Light: Corallife 6700K T5-BP
Sand substrate
2 tall pieces of Driftwood

PH: 6.4
Temperature: 84F

The tank consists of RO water since our tap water is extremely high in TDS and chlorine.
We did a 25% water change last Wednesday with 100% RO and I sifted through the sand to avoid toxic gas build-up.
Today, we would like to do a water change again, this time with 80% RO and 20% tap water. We conditioned the tap water and kept it in a barrel with an aerator over night. We also will run the tap water one time through a filter (with carbon this time) before we use it for the water change. Does that sound OK?
There is quite some algae on the sand and on one of the driftwood pieces which I try to vacuum during the water changes.

My other questions are:
Can the algae be responsible for a suppressed appetite? Do we need another UV sterilizer or does it just take more time to do its job?
Is it safe to sift through the sand during every water change?
Are two water changes a week enough?
Is the 80/20 ratio of RO and tap water OK?
Do we need more aeration?

I am feeding pellets in the morning and sometimes at lunch when I can't stand the begging. Late afternoon I feed frozen bloodworms. My little problem-fish is not even interested in those. The others seem "normal" to me, but again, we are new to discus and not quite sure what "normal" is. They quietly swim around or hang out in one spot, chase each other and come to the front when I approach.

Is there anything we do wrong or are not thinking of?

We are heading out now to our LFS to get a new testing kit to test ph of our RO and tap water (out of faucet and out of barrel). Maybe that will shed some light?

Any advice would be greatly appreciated!
Thank you!!!!

[adapted]

If you have a problem with algae, your lighting may be too bright for the fish. My wilds are much happier when the light is as subdued as possible.

Your water changes are probably insufficient. More is almost always better. Since you haven't have discus long enough to know their behaviors, I would up the amount and frequency. You can back off later if you think you're overdoing it. I do a 50% change every day for 8 adult wilds in a 120 gallon tank. Settle on an R/O - tap mixture and do that regularly.

Discus can go quite a while without eating. Don't worry too much... just keep observing and tempting him with the bloodworms. It is more ikely to be stress (both pecking order and changing conditions) that is causing the problem rather than something you need to be medicating. If he's still not eating in another week or two, you should still have plenty of time to try a medication. For now, work on making the water and the lighting better.

[discus_gal]

Thank you very much for your response, adapted. We did a 35-40% water change with about 50% tap and 50% RO. They look a little lethargic but hopefully perk up again soon.
TDS now are 168.4.... quite a spike from the previous 40. :(
I guess we have to do w/c more frequently, like you said and see what happens. I understand they need the minerals of the tap water, but the TDS in ours is just so ridiculously high (900 out of the faucet).