High tech planted tank
For evaporation Iíve been topping off with tap water with Prime but I read how this is not good because TDS will keep building up?
I do have a RODI 5 stage unit for my saltwater tank, is it better if I use RODI water for top off? I was under the assumption that tap water with prime is better because of the minerals and trace?
Evaporation will leave minerals behind in the aquarium (only the water evaporates). Topping off with tap water will increase TDS.
Using RODI is generally recommended for this purpose, but it depends (on how much evaporation is occurring, how often you change water, etc)
I would not use tap since you have the RO water on hand for the reason Darkblade48 stated above, you will have a buildup of nutrients, using the RO water will keep the nutrient levels more stable as is what you want in a high tech tank, stability.
We dose our own traces(or at least we should be), so relying on your tap to provide them is not a good plan. If you're relying on your tap water to provide ANYTHING other than H2O, you need to take a close look at your dosing and reevaluate.
As the previous posters have said, you have the RODI available, use it for top offs. Otherwise, you'll start to get TDS creep and you can see your GH and KH start to increase over time.
Sure evaporation is going to leave minerals behind. Yes your tap water might cause TDS to increase. But that doesn't necessarily mean its a problem or going to wreak havoc in the tank. Everyone's parameters are going to be different so there is no steadfast rule to use RO water for topoffs! Its a waste of time and money for most people's tanks whether they're high tech or not.
Blindly suggesting to use RO/DI for topoffs without knowing more details about the OP's tank, or water parameters, or tap water parameters borders on crazy IMO! I guess there is no harm in using it, but there might be benefit for certain circumstances as well.
The things that make up the total TDS in your water are probably not the same dissolved solids in my water.
OP said he loses about 500 ml per day in a 17 gallon tank which works about 1 gallon a week and does 35% water changes weekly. So at the end of the week (without topoffs) there is roughly 16 gallons of water left and he removes roughly 5.5 gallons during a water change. The evaporation seems minimal and unlikely to cause any huge parameter swings.
How long has this tank been running and are you encountering some sort of problem that you think could be caused by using your tap water for top offs? If there is no problem to be seen then there is no reason to try and fix it! [