Although I'm not the best deck-builder in the world, here's my advice:
Don't add a third element. Trying to toss in Water creatures that may use more or less
![Water :water](https://elementscommunity.org/forum/Smileys/solosmileys/../../../images/Misc/water18x18.png)
quanta can be hard to balance quanta-wise, so I wouldn't even try. If you're focusing on the synergy between Death & Darkness as the basis for your deck, keep that in the forefront of what you want to improve on; don't weaken the elemental combo by adding more elements to do other things such as 'stalling' CC and whatnot.
First off - ditch ALL of the viruses. Why? Because a one-turn infection may not make a difference against non-upped creatures with high HP, and you already have Parasites that provide that CC.
Second; add a couple of Soul Catchers (if possible). You can increase your
![Death :death](https://elementscommunity.org/forum/Smileys/solosmileys/../../../images/Misc/death18x18.png)
quanta generation that way when you kill critters using the Infect ability from the Parasites.
Third; replace one Bone Wall or a Boneyard and a Parasite with two Nightfall cards (as your suggestion earlier) to increase your damage. The final result should look something like this:
52g 52g 52g 52g 52g 52g 52n 52n 52r 52r 52r 52v 52v 5uk 5uk 5uk 5uk 5uk 5uk 5uk 5uk 5ul 5ul 5ul 5um 5um 5um 5um 5um 5um 5un 5un 5un 5un 5uq 5uq
If this doesn't work, feel free to ask for more help instead.
![Smiley :)](https://elementscommunity.org/forum/Smileys/solosmileys/smiley.gif)