Hi there, ornaments are fine as long as they are from a reputable company you can trust has tested for toxins that may leach into the water... eBay is fine if there is a tag attached from such a company ie. TopFin.
A few things I want to point out you need to be careful of:
1) your substrate... It must not have shells or corals, it will raise the PH, make sure it is inert. Exact same for rocks... Limestone raises ph as well, they must also be inert. discus
thrive in acidic to neutral ph, anything Alkaline is not ideal and they will be stressed. Mine is 7-7.2 out of the tap and aged 24 hours and my discus
breed and thrive, any wild discus would not, they require more acidic water. Yours are hybrids and not wild strains therefore ok.
2) your filter looks way too small, not enough media to sustain bacteria for that kind of bio-load. EHEIM canisters are exceptional or aqua clear hang on back, a 70 or 110 would be fine on a 55. Cut a plastic water bottle lengthwise to lay in the filter at the overflow and the current will be reduced for discus. They need calm flowing water.
3) water changes...every day is best. If you want health, growth and vigor, waterchanges are your answer. Heated and aerated and aged in a barrel 24 hours will show results. Unless your tap water is extremely alkaline, treat with prime or chlorine/chloramine/ammonia etc remover and change as much as you can. I do 40-50% every other day and my fish thrive. My baby grow out bare bottom tank gets 90% aged water every day.
4). If you're using led or t5 HO lighting like I am, make sure it's well planted with tall plants to give them shade and cover, they don't like bright lights and will need to feel secure in tall grasses etc. just like in the wild
5) the tannins in any aquatic driftwood will release and go away soon enough with water changes. If you're doing waterchanges once a week, how much are you doing? Especially now, to avoid ammonia and nitrite spikes that will damage the gills of your fish and possibly kill them, you need to check levels and do frequent waterchanges. It takes at least 8 weeks for the nitrogen cycle to establish enough to process ammonia into nitrate... Stability by seachem will help plant some beneficial bacteria #1 into your filter, but time is the only way to build the second nitrifying bacteria. That's why people use fish food, pure ammonia, or sacrifice fish to produce waste to get the nitrogen cycle established before adding fish.
I hope that helps a bit and can get things settled so your aquarium will continue to provide calm and relaxation just as mine do