Generally the biggest problem encountered when using tap water is algae problems that cannot be beat. The source of the algae is the water you are putting into your system as tap water often contains high levels of nitrates and phosphates, which is fuel for algae. If the water contains silicates, you will have a diatom problem that won't go away, since diatoms use silicates as their fuel to build their shells, and the tap water provides them with an unlimited fuel source.
Another common problem is heavy metals in the water, especially if your house has copper plumbing. Copper is fatal to inverts even in the smallest concentrations, so some people have found it impossible to keep anything but fish alive when using tap water, since the copper in the water kills corals and inverts.
Another problem is making your own saltwater at home. Marine and reef salts are balanced and meant to be mixed with pure RODI water, not tap water that already contains a lot of stuff in it. Making up your own saltwater at home from tap water can lead to things being out of whack. Probably not a big or common problem, but it happens.
This is probably not what you want to hear, but your girlfriend should know that an RODI unit is one of the most essential pieces of equipment in this hobby. You get out of it what you put in, and if you put garbage in your tank, you get garbage out. Starting off the tank on a good foot with good water is essential. You can do water changes all you want, but if you are using new bad water to replace old bad water, it accomplishes nothing. You can get an RODI unit for around $125 online.
Water conditioner will remove chlorine and chloramine, but will not do anything for the algae-causing nutrients or heavy metals found in tap water.
Phosphate tests are pretty inaccurate. If you have algae problems, the algae will often consume the phosphate as fast as it is produced in your tank, leading to false readings of zero. You can have a terrible phosphate problem but your test kit will tell you the level's at zero, and your algae is growing out of control.
So you say you will be doing 5 gallons water changes every 5 days, and buying that water from Walmart eventually. It's $1 a gallon at Walmart. That's $5 in water every 5 days -- or $30 in water for water changes each month. You will have made up the cost of a $125 RODI unit in a few months. And that doesn't even include the top off water you will be replacing due to evaporation -- which is several additional gallons a week (in the case of my 90 gallons, it lost about 5 gallons a day due to evaporation). I bet you'd have to add in another $10 or so a month for top off water, and then you are spending more at Walmart within a couple months than had you just bought your own RODI unit and made water yourself from the start. It really doesn't pay to buy your water at the store, it's cheaper to make your own.
