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Should I do a small water change near the end of cycling my dry rock in my marine aquarium?

Ammonia levels are 0, nitrites are at 0 or very well close to 0. But nitrates have been explosive over the last few days. There is nothing in the tank, my dry rock that I've been ghost feeding and seeding for about 3 1/2 weeks, and my live sand.

I saw ammonia spike, then come down, nitrites spike then down. And now I'm just worried that these high nitrates could start blooming algae or something. There is no light on the tank, except the occasional flicked on and off when someone ventures into the basement where the tank is located (no natural light at all).

Am I safe to do a small water change, I was thinking 10 - 15 % to help reduce the nitrates then I was going to start my skimmer. I'm fairly new to the hobby as this is my second and far more researched attempt after an 8 year break.

Additional tank info:

  • size: 75 gallon (280 liters);

  • rock: 75 lbs (34 kg) from bulk reef supply;

  • sand: 40 lbs (18 kg) live sand;

  • temperature: 27 °C (80 °F);

  • salinity: 35 parts per trillion, specific gravity of 1.026;

  • skimmer is currently not running during cycling;

  • lighting is currently off as well.

Using RO/DI system with out water reading 0 total dissolved solids (mixed and heated in a storage container for all my water changes).

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So, I'm going to assume you're curing/cycling the rock in the display tank, correct me if I am wrong here.

Assuming so, I would definitely do a water change. And I think your concerns are spot on. As soon as you apply light of any reasonable intensity, you're going to almost immediately get GHA and film algae, typically followed by more nefarious algaes. Right now is when I would figure out an initial water change and maintenance schedule. If you do decide to add livestock, do so extremely slowly because it's going to be months to a full year before the tank has any chance at natural denitrification. The 2 main concerns in adding too much are creating runaway NO3- and PO43- which will turn into a lot of maintenance to keep algae in check. Secondly, if you add a lot right now, it's possible to overload the biological capacity of your rock and substrate and cause what's called a soft cycle, which is basically a mini-cycle that lasts from about 1 - 3 weeks. You won't get near the ammonia spike of an actual cycle, but it can be enough to stress or kill fish and definitely enough to kill coral or inverts.

There's isn't really a correct way to handle this. Almost all reef tanks are going to go through a number of algae stages the first year they are setup. The goal is to minimize the carnage during that time and do everything you can to not introduce aggressive algaes like bryopsis or caulerpa racemosa among others. Just basing this on my experience, I would actually plan for roughly 10 - 15% per week for the immediate future. If you stock no fish or very lightly, you can extend weekly to every 2 - 3 weeks, dependent on whether you start having nutrient problems (NO3- and PO43-). Once the tank is several months old, and you have a routine schedule, you should be able to incrementally adjust it to meet the needs of the tank.

Also, it's not uncommon at all for dry rock, including mined rock like BRS pukani, marco, or reefcleaners, or others to leach PO43-. I personally do not have an explanation for this phenomenon, but I've seen it in my tanks and almost every other tank I've seen setup using dry mined rock. Keep an eye on your phosphate, as it's basically the other contributor to nuisance algaes, cyanobactera, diatoms, etc. If you have a chance, I strongly recommend joining a local and/or online reef forum or group. This is truly a community hobby, and you can learn a lot more and meet really great people through one of these clubs.

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  • Thanks for the information! I've done a lot of research this time around, but the second I think I have it figured out a different forum will say some different information. I come from an IT background and joining a 90s looking forum to become part of the "hobbyist community" is scary to me. Most of them are dated to 10+ years ago and look pretty stale. I've still pulled good information from many of them, but would you recommend a site that might be more up to date and formal. That's actually why I'm here in the first place.
    – Fus Ro Dah
    Commented Mar 17, 2017 at 14:33
  • fwiw, I wasn't planning on adding any livestock for even the next few months until things were stable and I was going to get a clean up crew (probably room for a different discussion). I have a pretty complex setup aimed at ease of use for water changes so the frequencies of water changes are no issue to me - I'll have a full tank and a half of warmed saltwater ready each week.
    – Fus Ro Dah
    Commented Mar 17, 2017 at 14:36
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    nano-reef.com is a good one that's not stuck 30 years ago. For large forums reef2reef would probably be my second pick. I've had enough mixed experiences on reefcentral, it's one I rarely visit. Not sure where you're located, but many areas have regional forums that are very active and current. Best bet is to search online or if you have any good local fish stores, they are often sponsor or are very knowledgeable about local clubs. Local clubs are usually by far the cheapest places to get coral as well. I can typically undercut good local and online stores by 50%+ getting from other hobbyists.
    – Jestep
    Commented Mar 17, 2017 at 17:05
  • Otherwise, I think your plan is solid and shouldn't bring up any major issues. Usually people are way too quick to want a full blown reef, and it's just not something that can be rushed.
    – Jestep
    Commented Mar 17, 2017 at 17:07

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