oh no lol why have i made a mistake lol
LOL, just a couple things and then the rest will give you full explantion lol, discus should be kept in much bigger tank and larger groups to even out aggression and keep then more secure (less spooked) and need very good water conditions and higher temps.... angels need at least 17inch deep tank.... you may find the neons will be a snack for the angels when they get larger ... think thats it for my quick explanation lol
Not to mention that Silver Shark require a group also, and get as long as the tank they are in now
A lot of the fish you have aren't good stoking wise, and most will likely need re-homing I'm afraid

IMO, Angels are just about OK in a record 98 if you don't have too much decor, but you need laid back angels for no decor to work... the Silver shark are too big and have to go unfortunately. Discus likewise, too big for a 98l in the minimum group, though a proven par will be fine in a 98 record. they'd be the
only fish in there though, they'd literally kill everything else when spawning...
Neons are a good choice, and can would long-term with angels IME. Just don't add to the group once settled. the angels grow up with the Neons and don't regard them as food. they get used to you opening the flap and adding dinner. When you open the flap and add a bag of new Neons, they think "great, live food!" and suddenly regard both the new and existing Neon stock as lunch and thing start to go down-hill rather fast from there for the Neons...
What "Algae Eater" do you have? there are many fish under this name. Some are OK, many are not
I'd let the UGF (Under Gravel Filters) alone. They are the best filters for Mechanical and Biological filtration, but very high-maintenance and best suited to more experienced aquarists whom know all the ins and outs of this "old school" technology. Many have issues maintaining them

If you want better filters, take a look a Externals, but when it comes to internal filters, the Juwel internals are good. I've taken the Record 98's internal through hell kicking and screaming backwards, before dragging it back again and I've never had an issue with it
I don't suppose you can post some water test results (numerical preferably) can you, along with the make and type of test kit you have?
All the best
Rabbut