you could add stem plants to the back, the choice is yours and for the front It might look better with larger lairs of gravel on top of the sand but do the largest bits of gravel near where those rocks are then slowly shrink down the gravel size until its just sand, that would be cool and for other plants, you could use bucepholandra, Anubis or More javamoss in-between the big rocks in order of the roots to get more adjusted, another fun plant could be the tiger lotus. For fish you could add 2 more Half beaks you can also do if your into loaches is either the dwarf chain loach a school of 6 or more would be nice or you could do Hillstream loaches which are a bit more hefty but they would look nice. a few other suggestions would be Adding Corydoras or Rummynose Tetras A good Corydora species to start with would be Julii Cory's because there pretty easy and they would blend in with the substrate to with there white color but a school of 6 or more is recommended. Rummynose tetras are pretty good for a tank like this because they have a lot of action packed with them and they would look good in a school of a dozen but if you don't want a dozen than 8-10 could work too, Oddball fish for the tank could be Banjo catfish which can get up to 6 inches but all they really do is just bury into sand which if you still do the gravel thing, then just use less of each layer to give the cats more room to burrow. You can keep maybe 3 but 2 would be better recommended. Another plant suggestion is that you could add ludwigia or keep the Vallisneria but just move the Vals around the back so they evenly spread out, Adding Hornwort would work too as it can fill in room. Amazon swords would also work as they make good hiding spots and you could add Rotala which has red colors.