Help to Create Brackish tank for "Freshwater" Flounder, Please

[FreshFlounder]

Hello. I have a 14 gallon freshwater biocube. Three years ago I got two "freshwater" flounders at about 1/2" in size. Substrate is soft sand. Water temp fluctuates between 70 and 75. Phil and Floyd did great. They were best friends. They are only occupants of tank. They seem to only eat frozen bloodworms. I offer other frozen and sinking foods, but I only saw them eat bloodworms. Floyd was always the fat one. He died a week ago and had no obvious prior symptoms. Ate well the night before. I came home from work and he was dead. My API water parameters were great but I did a partial water change and changed filter. I am now down to Phil. My previous research apparently was weak. Recent research that I have gathered suggests that they should be at brackish water as adults. I believe they are Trinectes maculatus. I read that they prefer 1.005 and 1.010. I bought an Instant Ocean hydrometer. I am trying to get a Koi medic salinity reader for my birthday. My tap water (which I dechlorinate and use for my tanks) measures 1.003 and flounder tank measures 1.004 currently. It has been suggested that I increase salinity by 0.002 per week until I reach 1.010. I can't seem to figure out how to do that. I have tried some online calculators, but they suggest 50 one gallon water changes to change 0.002. I must be doing something wrong.
1.)Does anyone have any experience with these flounders? Their ideal husbandry?
2.) How can I safely raise the tank salinity? Specific help is appreciated, like "remove one gallon and add one gallon of water that has 1 tablespoon of Instant Ocean marine salt dissolved in it. Repeat this weekly until your goal is met. Measure salinity before and after addition of new water." I feel like this is rocket science to my feeble mind. I can't seem to figure this out.
3.) Once I reach ideal salinity, what mixture should my water change be when I clean tank? I am assuming same salinity as tank?
4.) Does anyone know if juvenile flounders can be kept in brackish water? I was going to get a friend for Phil but the fish stores keep them in strict freshwater. So I was going to raise the salinity gradually while it was in quarantine.
5.) Is tap water (city water) ok to dechlorinate and use for brackish tanks? Any ideas why water is 1.003?
Any help is greatly appreciated! Thank you! Sincerely, Amy

 

[OrQidz]

Welcome to RC! Sorry to hear you lost Floyd. That's always a bummer. In regards to brackish: yes I think dechlorinated tap as you've been using is fine. I'm not sure why your tap is showing 1.003 but it's quite possible it's not accurate. Those swing arm hydrometers can be inaccurate. If you've just filled it there will often be tiny bubbles on the arm which make it look like the salinity is higher than it really is. Let it sit for a while and tap it to dislodge air bubbles. Getting a better one as you are hoping for is a good plan.

To change over to brackish you want to go slowly. A sudden change will be a shock to Phil and your biofilter. The no more than 0.002 per every 1-2 weeks is the rule I've heard also. So, take a gallon of water and dechlorinate it like you normally do. Add a tablespoon of marine salt. Let it dissolve and sit for a couple hours. Check the specific gravity. Take a gallon of water out of the tank and replace it with the slightly salty water you've just mixed. Do that once a week and the tank's salinity will slowly increase.

Once your tank is at the desired salinity, your water change water should match the tank. Use something to measure the salt, don't just eyeball it, so you're consistent.

It's not rocket science, don't worry. You'll see once you get the hang of it. Good luck with it!